It starts with a drunken bet. Chris and Ray are slumped over the bar at Luigi's while Ray stares unabashedly at Luigi's niece Ana, a fag hanging from the corner of his mouth which has long burnt out.

"I like them Italian birds," he says, nodding his head in Ana's direction. "They're all curvy, aren't they? Must be something to do with the food they eat."

Chris frowns. "What, like pizza and stuff? Our Shazzer eats pizza and she don't look like that." He glances over at where Shaz is sitting in the corner, flicking through a magazine while she listens to her Walkman. She's bobbing her head in time and her dark hair falls into her eyes with each beat.

"Nah, not like that, mate. Proper Italian food. I dunno, I don't speak Italian. I'm telling you though - go to Italy, and they all look like that. I reckon I'm going to try and retire there." He lets out a breath of appreciation as Ana bends over to set two wine glasses on the table DI Drake and the Guv are sharing. "Would you look at those? Cracking pair of puppies."

Chris stares blearily into his beer, noting vaguely that there's something floating at the bottom of his glass. "Shaz says it's wrong to objectify women, and that's it's the inside that's important, not the outside."

"Exactly, mate." Ray waggles his eyebrows. "It's what's under them clothes that count, and from what I can see, that Ana is shaping up nicely."

Chris starts to reply, and then frowns. He doesn't think that's what Shaz meant, but he's had one too many pints to bother checking with her, and he's not sure Ray is listening anyway. "Careful, mate, your eyes'll pop out soon, you're staring at her so hard. She's going to notice in a minute, and then Luigi'll chuck us out."

"Shut it. You're not one to talk about subtlety. Last time we had to follow that burglar bastard you knocked those bins over and he bolted." He shakes his head, finally relighting his cigarette. "Twonk."

"Yeah, well, who leaves their bins in the middle of the street anyway?" Chris gets a fag out of his jacket pocket but doesn't light it, just jabs it at Ray in a vaguely threatening way. "I could be a proper good spy, I could. I'd be a whiz with them gadgets. I could even fool..." He casts around for inspiration, eyes finally settling on the corner table. "...the Guv."

Ray snorts. "You've got about as much chance of trailing the Guv and DI Drake as I have of getting into Ana's Catholic knickers." He pauses for a second, and then looks Chris in the eye. "Go on then, mate, let's put a little wager on it. You've got a week to find out whether the Guv and DI Drake are shagging, and I've got a week to try and get Ana into bed. First one to come up with the goods wins."

Chris considers this for a moment, frowning. "Why are you so concerned about the Guv and DI Drake? There's no way they're shagging."

"I'm not telling you anything, Columbo. That's your job. Whoever loses has to buy the drinks for a month." He pauses. "And do the paperwork."

Chris knows, somehow, that this isn't a good idea. For starters, he's got no idea how to lead a surveillance operation, and he's not even sure he wants to know the ins and outs of the Guv's relationship with DI Drake. But then he thinks of Ray's face when he loses, and Shazzer's pride when Ray is forced to buy them both a drink.

And suddenly, without really knowing why or how, he finds himself shaking Ray's hand.

"I can't believe we're doing this." Shaz is sitting beside him in the car, hunkered down, face hidden by an enormous pair of sunglasses, but he can hear the annoyance in her voice. "I've told you, it's obvious the Guv and DI Drake are in love, and any snooping on your part is just a stupid way of getting Ray's approval." At the look on his face, she softens, reaching out to touch his arm. "Sorry, baby. It's don't need a caveman like Ray to pat you on the head. You're a good copper, and he knows it."

Chris shifts uncomfortably, cracking the window open a little to let in the breeze. "All right, Shazzer, I'm not copper of the year or anything." He tries to hide a pleased smile. "Don't you want to see Ray doing all my paperwork for the next month? If the Guv and DI Drake are as loved up as you say they are, it'll be a piece of cake, won't it?"

Shaz sighs, leaning forward to fiddle with the cassette player. "I still think you're mad, but it's up to - Chris! They're coming!"

There is a brief second of blind panic, where Chris freezes with one hand on the steering wheel and the other on his binoculars, and then Shaz is grabbing the back of his head and pulling him into a kiss, faces so close they are indistinguishable. He opens his eyes just in time to see the Guv and DI Drake disappear into Luigi's, and Shaz lets him go, breathing heavily.

"Bloody hell! That was one hell of a kiss!"

She grins at him. "I'm going into Luigi's. Park the car and follow me in."

She's out of the car and down the steps before he knows it, and he wonders whether it was such a good idea to get a bird involved, after all.

By the time Chris finds a parking space and walks into Luigi's, the Guv and DI Drake are already sitting at the corner table, watched avidly by Shaz, perched on a stool at the bar. Their heads are bent close together, the Guv's chin propped on his hand, and there is the tiniest hint of a smile on his lips that makes Chris pull up short.

"The Guv..." he whispers, taking the seat beside Shaz. "He's smiling."

Shaz gives a smile of her own, one that is decidedly more self-satisfied than the Guv's, and looks at him smugly. "Told you, didn't I? Like two pieces of the puzzle, they are. It's ever so romantic." She sighs, twirling the stem of her wine glass in her fingers. "How are you going to get proper evidence, anyway?"

Chris looks up from where he is hastily scribbling in his notebook. "Log of sightings," he says, tapping it with satisfaction. "And witness statements." He starts to write again and then pauses, nods at her. "That's you, like," he adds, just in case she didn't understand, before turning back to his notes. Friday, 7.46pm - the Guv and DI Drake making eyes at each other.

He sets down his pencil and sips ostensibly at his beer, eyes fixed firmly on his superior officers. The Guv leans over and for a split second Chris thinks he's going to kiss her, but his mouth moves against her ear instead, whispering words meant only for her. From the blush that steals up her neck to colour her cheeks, Chris can only guess at what he's said, but he looks at his drink with satisfaction. Next week, this will be on Ray.

He picks up his pencil again. Friday, 7.51pm - the Guv says something dirty.

"Right Charles Dickens, you are," Shaz remarks with a glance at his notebook, and he frowns.

"Shut up. It's only Ray that's going to be reading it. Doesn't have to be bloody Shakespeare." He knows he's being tetchy, but there's something nagging at the back of his mind. This doesn't feel right. Not like a quick in-out-off shag, like Ray was hoping. The way they're sitting makes Chris feel a bit like he's intruding, watching them like this. It reminds him of how he and Shaz sit on the sofa, close without meaning to be, as if somehow they're drawn together without even thinking about it.

Luckily, the phone rings and Luigi answers, summoning the Guv with one hand. "Signore Hunt, a phone call for you." He hands over the receiver and Chris pretends great interest in his beer in an attempt to feign nonchalance, and he realises suddenly that Shaz is talking to him as if they'd been in the middle of a conversation.

"So I said to her she should just ditch him. I mean, what does she want with an idiot like that?"

The Guv turns to look at them over his shoulder, his earlier smile now replaced with a fierce scowl. "As much as I hope you two are enjoying your mothers' meeting, some of us have got police work to be getting on with, so shut it."

He turns his back on them again and listens intently to whoever is on the other line. "What's that you're saying, Skip? A body? Bugger it. Give us five minutes." He hangs up, cocks his head at DI Drake and then nods at Chris and Shaz. "Come on, Donny and Marie. There's been a murder."

Chris likes to think he's got an iron stomach, the amount of gruesome sights he's seen over the years, but even he feels nauseous when they reach the body, spread-eagled across the pavement like some failed attempt at flight. The PC guarding the scene looks to be barely more than a teenager, his face blanched, eyes forward as though he can't bear to even consider the sight behind him.

"Off you go, PC Plod," the Guv says, clapping him on the shoulder, and the young policeman scarpers. Chris can't help a fizz of envy as he watches the man drop into a police car and zoom off around the corner. Corpses have always given him the heebie-jeebies.

"Bloody hell." Ray is standing over the body, a fag hanging out of his mouth. "That's Jimmy Burrows, that is! Bloody nonce got himself killed before we could get to him."

Curiosity peaked, Chris edges around the pool of blood and peers at the man's face. Ray's right – beneath the bruising and bloodied mouth, it's clear that the body is that of Jimmy Burrows, a lowlife drug dealer they've been after for weeks. He steps gingerly away, wiping his shoe against the curb and leaving a trail of blood.

"One less scumbag for us to catch, anyway," Ray says almost cheerfully, relighting his cigarette and exhaling a stream of smoke into the evening air. "He was getting on my tits, he was, what with all that changing-his-name business."

"For Christ's sake, Ray!" DI Drake speaks for the first time and Chris jumps at her tone. He'd never tell Ray, but he finds her a bit intimidating. She's been to university and everything, and she's been putting ideas into Shaz's head, ideas that make Chris a bit uncomfortable. After all, he doesn't want Shaz becoming a sergeant before he does.

"All right, Ma'am, calm down." Ray gives her a funny look. "He's only a druggie."

Chris notices suddenly that the Guv hasn't said anything. He's watching DI Drake very carefully, and there's a funny expression on his face, one Chris hasn't seen before. It's like he can feel what she's feeling, like he can read her mind or something, because Chris has known the Guv a long time and he wouldn't get upset over a dead druggie.

"He was sixteen! A kid, Ray. At school still. I only spoke to his mother last week!" She turns abruptly away and Ray makes a face at Chris behind her back. He mouths something - bloody women – but Chris is too distracted to bother replying. He's just realised that this could turn into an opportunity for surveillance, and he wonders vaguely whether it'd be too obvious to get his notebook out. He gives Ray a sideways look but he's turned away, eyes fixed on a blonde woman crossing the road in the distance.

Chris clears his throat. "I'll, er, just have a look over here, see if he dropped anything when he fell," he says loudly, walking pointedly away from the Guv and DI Drake. He smiles secretly. And people think he can't be discreet.

He stands by someone's garden gate, poking around theatrically in the shrubbery while his eyes are fixed on his senior officers. The Guv looks around quickly, checking that both he and Ray are occupied, before taking a step closer to DI Drake, bending his head so that only she can hear his words. She shakes her head, pushes him away with one hand, but he catches her wrist, holds it so lightly it's as if he fears she might break.

Chris makes a mental note. Guv holds DI Drake's hand. It's strange, the way this scene is playing out. Chris has only known the Guv as strong, hard, but the way he is with the Boss...he's like a truffle, cracked open to reveal a soft heart. It makes him uncomfortable.

It is only when DI Drake starts to cry, her body crumbling like a leaf in the rain, and the Guv holds her gently against him, her face tucked into the cradle of his shoulder, that Chris coughs and goes after Ray. When he glances back, she is still in his arms, and the Guv's eyes glint silver-blue, gazing sadly over her shoulder as though he's looking into the past.

"Signore Carling, if you touch me again, I will empty this jug of water over your head!"

Ana's irritated tones drift across the restaurant to where Chris is propping up the bar, and he grins contentedly. His evidence on the affair of the Guv and DI Drake is growing by the hour, while Ray seems none the closer to getting into Ana's knickers.

Just then, the man in question appears, cradling a pint in his hand as he winks at Chris. "Told you, mate. She wants me."

From her position on the bar stool beside him, Shaz gives him a look. "Oh give over, Ray. She just told you she'd dump a gallon of icy water on your head if you went near her again! You're going to get us barred at this rate."

Ray shakes his head smugly. "See, what you don't understand, Shazzer, is that this is all a game to her. She's playing hard to get, like, but she won't be able to resist forever - you'll see. It's called sexual politics."

"It's called harassment, that's what it's called," she snorts.

"Yeah mate, think you'd better start saving your pennies if you're going to be able to afford to buy my drinks for the next month," Chris chips in, trying and failing not to look too self-satisfied.

"Oh aye? How's your surveillance going then, Sherlock? You're just bluffing, I can tell. Never did have a good poker face."

"I am not! I've got loads of evidence, I have. I've got Shaz as a witness and everything." He takes a glug of beer. "Proper surveillance. Told you I could do it."

"Well, that doesn't count!" Ray exclaims derisively. "You can't use your bloody girlfriend as a witness! She could be lying through her teeth just to make you look good."

Shaz sighs. "Believe me, Ray, I've got better things to do with my time than waste it spouting rubbish with you two." She gets up and walks over to join a gaggle of WPCs at a nearby table and Chris watches her go, faintly confused. He's not sure if she's angry at him or whether it's just Ray who's getting up her nose, but he doesn't want to go over and talk to her again in case Ray calls him a poof.

"Women." Ray shakes his head, scowling. "Don't know how you put it up with it, mate. All that nagging. Does my head in."

"Why are you so bothered about sleeping with this Ana then? If women are such a nuisance?" Chris wonders if he's drunk too much or if Ray's just being a div. He tries to count the number of pints he's had. One, two, three...

"That's different, mate." Ray wiggles his eyebrows. "Nothing wrong with a bit of how's-your-father, especially not with someone as ...well-endowed as her." His eyes follow Ana's cleavage as she bends down to set a tray of drinks on Shaz's table and then he meets Chris's gaze, licking his lips.

"I dunno...Shazzer says that sex is only truly meaningful when it's with someone you love."

"Oh Christ." Ray shakes his head in disgust and gets to his feet. "I'm not going to sit here with you talking sentimental bollocks like that." He picks up his glass. "I'm going to find Ana."

Chris watches him go, eyes travelling slowly around the room. Terry and Bammo are playing darts, so drunkenly and wildly that Luigi has to duck every time he scurries past with a tray, and Poirot is re-enacting a collar from that afternoon, much to the amusement of Viv and the plods gathered around another table. Suddenly, his gaze lights on the Guv and DI Drake, heads bent together, knees touching.

He immediately perks up, fumbling for his notepad and then sighing as it skids across the counter to land in a pool of spilt beer. He squints at the pages. The ink has run a bit but it's still pretty readable, in his opinion. Anyway, once Ray sees how many notes he's got, he's hardly going to quibble about a beer stain or two.

Taking out his pencil, he makes a quick note.

9.48pm - The Guv and DI Drake snuggling.

Oh, bugger. He realises belatedly that the word 'snuggling' makes him look like a poof, so he strikes it through twice and then scribbles over the top for good measure.

9.48pm - The Guv and DI Drake touching.

Their sombre mood of the past few days has lifted since finding Jimmy Burrows' killer, but CID has been quieter, with fewer rows, fewer raised voices. To Chris's mind, it's like bright blue of a sky after a storm, when the clouds have been temporarily scattered and all that's left is a vista of possibilities. He frowns at his glass. Blimey, he must have had more to drink than he thought, if he's spouting this poetic drivel.

A movement on their table catches his eye again and he forces himself to focus. DI Drake's hand has slipped beneath the wooden surface to stroke lightly along the Guv's thigh, and the Guv is gritting his teeth so hard it's as if his jaw has been fused shut. Chris's eyes widen.

9.54pm – DI Drake gropes the Guv.

There's something mesmerising about watching other people, Chris realises, as events unfold before him. DI Drake is smiling, a tongue-in-teeth, teasing smile, not the one she uses when he's being thick, or when she's trying to comfort a terrified victim. It's a smile he hasn't seen before, a secret smile, and he realises with a jolt that it's meant only for the Guv. It's the same smile Shazzer gives him when he tells her he loves her, or when he's just kissed the breath from her. An intimate smile. The smile of a lover.

The Guv's hand comes down on DI Drake's beneath the table and she laughs breathlessly, so caught up in him that she doesn't notice Chris looking on. Instead she leans across to whisper in the Guv's ear and then gets up, squeezing past him so that he has an up-close view of her bottom.

10.03pm – DI Drake goes upstairs.

Chris looks at his watch and drinks his beer, counting the minutes.

10.11pm – The Guv follows her.

He smiles smugly and tucks his notepad away in his pocket. From across the trattoria, there comes a splash and then a stream of expletives.

"Jesus! What the bloody hell d'you think you're doing, you raving lunatic?"

Ray is standing opposite Ana, drenched in icy water, while she glares at him, empty jug held loosely in one hand.

Chris sniggers into his glass, and pretends he's not looking.

By Friday morning, Chris is feeling pretty smug. He's got a notebook full of evidence, he's made Shaz promise to back him up if Ray gets shirty, and he can foresee a month of free beer and no paperwork. He's already booked for him and Shaz to have the afternoon off on Monday so he can take her to the pictures, so confident is he of his victory, and he even finds himself whistling as he strolls into work and stops off at the custody desk.

"Morning, Viv."

"Morning, Chris. You're looking chirpy today."

Chris grins. "I feel it, mate. I've won a bet, I have. Ray's got to do all my paperwork and buy my drinks for a month."

"Blimey," Viv raises his eyebrows. "What did you bet on?"

Chris taps the side of his nose. "Couldn't possibly tell you. It's all part of a top secret investigation I've been carrying out. On the quiet, like," he adds, just in case Viv decides to bring this up with the Guv.

Viv laughs, pulling a list of registration plates out of a folder and handing it to him. "Seeing as you're on top form at the moment, Miss Marple, could you pass this on to DI Drake? She asked for it yesterday but it all kicked off in the cells and I couldn't give it to her."

Chris picks it up and runs his eyes quickly over the plates. He doesn't recognise any. "No problem. See you later."

He saunters off, still whistling, intent on giving the list to DI Drake first thing, before he has a chance to forget. He's lost count of how many times she's given him a bollocking for losing documents or failing to pass on information.

She's not at her desk, although Shaz tells him she came in an hour ago, and the Guv's office is similarly empty. He walks to the window and peers out towards the street, but the Quattro is still parked in its normal space outside the building, putting paid to any notion that they've already gone out. He stands still for a moment, stumped.

"What's the matter, mate? Surveillance not going well?" Ray sniggers, drawing on his fag and then exhaling jubilantly.

Shaz gives him a pointed look. "Trousers dried out yet, Ray?"

Ray scowls. "You can shut up, an' all."

Chris's gaze flicks between them, wondering if he ought to step in. Finally he does, unconvincingly. "Oi, don't talk to my girlfriend like that."

Ray gives him an incredulous look, as though he can't quite believe what he's heard. He shakes his head. "Twonk."

Chris hovers a moment longer, unsure whether he should just get on with some work until DI Drake returns, but the atmosphere is frosty all of a sudden. He half sits down and then hops straight back up again, gesturing vaguely to the door.

"Right, I'll just, er..."

No one is listening anyway.

He wanders back out through CID towards Viv, who claims he hasn't seen DI Drake all morning, and then bumps into Terry, who says he saw her heading towards the interview rooms half an hour ago. Chris takes his advice and takes a right, walking slowly along the corridor and peering through each window as he passes. The first two are empty. The third is occupied by Bammo, who's got a drug dealer pinned to the wall. It's in the fourth that Chris strikes gold.

DI Drake is sitting on the desk, leaning back on her hands, smiling up at the Guv as he moves to stand between her legs. As Chris watches, his hands come down to rest either side of hers, just close enough that their thumbs can intertwine. It's a gesture that's so intimate, so painfully familiar, that Chris suddenly feels awkward for witnessing it.

The Guv leans forward until his lips are a whisper away from hers, and then she tilts her head up, lets him kiss her with the casual ease of well-established lovers. It's this carelessness, this complete knowledge of each other, that Chris finds so astonishing, because it all seems so clear to him now and he can't understand why no one noticed it before. The way they are with each other, as her arms move to curl around his neck and his hands slide down her ribs to her waist, the assured touches, the tender confidence, it's breathtaking.

All of a sudden, Chris feels a bit like a pervert. He's not enjoying it, not like Ray might – he's actually (and he knows the boys in CID would never let him live it down if they could hear what he's thinking) revelling in the happiness on the Guv's face. He's known the Guv for a long time, and he's watched him sink low, lower than Chris thought it was possible to fall, what with the divorce and Sam's death and all the other shit that happens every day, but in that moment, he looks young again, young and free and happy. Blissfully, unashamedly happy.

And then Chris can't watch anymore. He steps back and walks away, leaves them to their kisses, to their secret laughter and hidden love. He returns to CID and drags Shaz out, leads her up to the rooftop and kisses her with the whole of London spread before them, the autumn sun casting copper highlights into her dark hair and setting her eyes alight with amber fire.

She doesn't ask what's brought it on, just wraps her arms around him and lets him hold her, head tucked into the cradle of his shoulder. He feels consumed by a rush of love, by the realisation that he's so much luckier than he'll ever truly understand, and he reaches down and laces his fingers through hers, marvels at the perfect fit.

And later that night, standing at the bar with Ray as he drains a pint of beer, he can't bring himself to tell the truth, to show him the notebook and lay their love bare on the table as though it's a worthless piece of gossip. So instead, he just shrugs, affects disappointment.

"Dunno, mate. Couldn't find anything...I don't reckon there's anything going on at all."

He never does throw the notebook away. Instead, he tucks it away in his bedside table and brings it out every now and then, when he's argued with Shaz about going out or the rent or, much later, the kids, to remind himself that true love is real, that it exists, that it's worth fighting for.

But on that night in 1982, all this still lies ahead, a future as yet unknown, a story still unwritten. So he endures the cat-calls and the mockery and the embarrassment as he has to pay for Ray's drink, and two storeys above, oblivious to the noise below, two lovers lay entwined, hearts beating as one in the peace of the night.