He erupts into my life on a sunny afternoon in early May.

I have a slight cold. I've had fish and chips for lunch with MacDee. And if I were a woman, I'd probably always remember what I'm wearing.

He wears a long dark coat. Far too warm for the day. He's odd in other ways, too.

On the campus of the Imperial College, Faculty of Medicine, I'm waiting in the corridor outside a dead professor's office. The door is closed and still taped off, and behind it, there is a mystery.

Professor Thomas Ellerton, a renowned cancer specialist in his late sixties, was found here yesterday morning by a cleaner, dangling from the lamp hook in the middle of his spacious office. Since then, we've gone over the room with a fine tooth comb - hours and hours of work, since it's in complete chaos. Someone has pulled all the books and folders and journals off the shelves. The floor is strewn with papers. His laptop is gone, too. Suicide, most of us would have said, if it hadn't been for the mess. And, more importantly, if it hadn't been for the utter lack of any object that he could have stood on when he put his neck in the noose. For a space of five feet all around that hook, there is a jumble of scientific publications on the floor, but no evidence of how he could have got up far enough off the ground to take the jump that killed him.

That is the reason why I'm back here today, waiting for the widow and her solicitor to arrive, and the private investigator that she insisted on involving in this. It's not as if she doubts our competence, the solicitor assures me on the phone when we make the appointment. But she's shaken to the core by her husband's death, and she feels that she owes it to him to leave no stone unturned to find out what exactly happened to him. I assure the lawyer in my turn that that's exactly what we will do, and that that's what we do best. But he isn't listening. He's probably the type who never does.

I relent, then. Half an hour in that office can't be too much of a waste of time. MacDee and I were going to go back there today anyway, in the hope of a brainwave. And by all means let them bring this private detective with them. Either he's a former colleague, as so many of them are, so he might even have something interesting to say. Or if he's annoying, I'll just throw him out, this being a crime scene and all. It's not like these guys don't know that they're on shaky ground at best when they try to compete with us. Not just in terms of their legal standing, but even more so in terms of expertise.

I've never been so wrong in my life.

I try to form a preliminary opinion of him when he comes walking - striding - down the corridor towards me, ahead - yes, ahead - of his client and her solicitor. Not a former colleague, definitely not, if only because there is not and has never been a man in the entire Metropolitan Police Service with that amount of dress sense. He's too young, too.

"Sherlock Holmes," he greets me, offering his hand. "We've met before."

No, we haven't. But then, that's what private investigators have to do when they get involved in a case from our domain. They depend on our goodwill, so currying favour by making friendly small-talk is part of the game. I can't blame him for trying.

If this is small-talk though, it ends here, and rather abruptly, too.

"You done in there?" he asks, pointing at the door to the dead man's office.

"Yes," I say, and he walks over and raises his hand as if to remove the tape. "I mean no," I correct myself quickly. "The forensic team's been and gone, but it's not been officially cleared yet."

He raises his eyebrows, and briefly, I wonder how he manages to make me feel that I'm the idiot here who's wasting a professional's valuable time.

"I sincerely hope that it hasn't been cleared yet," he replies. "Or how am I supposed to still find anything useful in there?"

And without so much as a by-your-leave, he does pluck off the tape, opens the door and steps in.

This is irregular, of course it is. But we actually are done here. The formal clearance is all that's missing. It's not like we're still expecting anything new to be gathered from the room as such, so preserving the crime scene uncontaminated in order to make any evidence admissible in court is no longer an issue. Not worth making a fuss about, particularly not if it helps the poor woman to some peace of mind.

Two or three steps inside the open door, Sherlock Holmes immediately raises his eyes to the hook in the ceiling, then turns back to me. "Where's the rope?" he snaps, and all but snaps his fingers as well in impatience.

"Removed, of course," I reply, irritated. "It's evidence."

And therefore already on its way to the lab, for traces of blood, DNA, or anything else that might tell us whether Professor Ellerton died a lonely death or not.

"Precisely, that's what it is," he replies. "So how do you hope to reconstruct events with any kind of accuracy if you take a natural entity apart like that?"

Next thing, he'll be complaining that we've taken the body to the morgue, too.

"Right, tell me what your theory is," he changes tack with dizzying speed, glancing around the room with those curiously pale eyes of his. Disquieting eyes. Cat's eyes, but for the slit pupils.

"Let me guess," he continues when I don't reply straight away. "You're currently inclined towards suicide, but you're baffled by the lack of any kind of physical support that the man might have stood on. Besides, the widow's statement that she made to your sergeant yesterday, in which she informed you that her husband was not mentally unstable, except for the occasional, probably age-related bout of absent-mindedness, makes her idea that this death might have been an act of revenge by a disaffected former member of his staff quite plausible, doesn't it?"

I haven't got past the first few words by the time he finishes that sentence. And he hasn't even drawn a single breath in between, as far as I can tell.

"Something like that, yeah," I manage to get out, and he smiles. Not a friendly smile though.

Yesterday, the widow lost no time telling my sergeant - Alec Macdonald, universally known in the force as MacDee - all about this man who was formerly an assistant of Professor Ellerton's. He was, by all appearances, himself destined for a brilliant career in cancer research. Until he got a little impatient and started fiddling with his test results to make them comply with his more spectacular theories. Professor Ellerton, when he found out, sacked him at once.

But the man now works in a private clinic in Portugal. As far as a first enquiry with our Portuguese colleagues could establish, he was present there at the time of the tragedy yesterday morning. Besides, that incident happened ten years ago. Would even such a humiliation still have rankled after all that time? And so badly that it would make a man embark on this kind of vendetta? Pull a fragile old gentleman up in a noose, and then go on to wreck his office, as if the killer wasn't only keen on destroying the man himself, but also, symbolically, his work?

There is something particularly ironic in that last idea, because the medical record that we got from Professor Ellerton's GP this morning states that he had, sadly, been recently diagnosed with incipient Alzheimer's disease. So not too long down the road, he would no longer have been able to impress anyone with his mental abilities anyway.

But the widow seems to have really got her teeth into the idea that this failed scientist must have had a hand in her husband's death. She even insists that her husband mentioned this unpleasant episode several times in the week before his death. If it was on his mind, she is convinced, it's a reasonable assumption that it was about to catch up with him.

"How high off the ground was the body when it was found?" Sherlock Holmes breaks into my thoughts.

"About a foot and a half. Not very high."

He rolls his eyes. "How high?" he repeats.

I sigh, and take out the case file from under my arm. "Seventeen inches," I inform him when I've found the relevant data in the reports.

A moment later, I realise that I've just given a private citizen, a casual bystander, a piece of confidential information out of the police file of an ongoing investigation. And I didn't even notice that that's what I was doing. How does he do that?

And what's he doing now? He's walked a few more steps into the room, squats down and starts to gather up the papers and books and journals that lie strewn all over the floor under the lamp hook.

This time, I don't only realise immediately that something's going wrong here. I also protest.

"Do you want this matter cleared up, or don't you?" he replies, not even looking at me.

I'm about to ask him why he thinks he's the man to do that, but then I admit that curiosity gets the better of me. He's clearly got some particular end in view, so I settle down to watch him, reminding myself that we were done here. He works methodically, with quick but very assured movements, outward from the centre, in widening circles, glancing at each of the documents as he picks them up, discarding some but collecting most of them.

When he's reached the outer perimeter of five feet in every direction - the same space where we failed to find anything that Professor Ellerton could have jumped from - the floor is almost clear. The papers he has gathered amount to an impressive stack. He gives it a pat with his hand and looks up at us from his place on the floor with an expression of great satisfaction.

"The academic achievement of a lifetime," he announces. "Every article, every scientific paper, every monograph he's ever published. The epitome and summary of everything he lived for."

I, the widow and her solicitor look back at him, utterly at a loss what he might be getting at.

He smiles again, and again it's not a friendly smile. "Oh, of course you wouldn't see it," he scoffs. "You people never see the wood for the trees, do you? Well, this is the wood."

He picks the whole stack up with both hands, places it right in the centre of the room, directly under the lamp hook, and squares and straightens it into a neat, solid block. Then he digs a measuring tape out of the pocket of his coat, takes a measurement, gets to his feet and holds the measuring tape up in display for us three to see, stretched out between both hands. The index finger of his left hand marks the height of the paper stack. Seventeen inches exactly.

With a click, the tape disappears back into its casing, and Sherlock Holmes turns to address the thunderstruck widow.

"Sorry to disappoint you, Mrs Ellerton," he says in a tone that doesn't express the slightest sympathy. "It was definitely suicide. So since you asked for my advice, I would advise you now to abandon the theory of murder at once. Unless you insist on turning this into a case of attempted insurance fraud, with yourself as the prime suspect."

The widow stares at him, aghast.

"But if, as I assume, you were unaware of your husband's intentions, you may take comfort from two facts," he continues generously. "The first is that your late husband was obviously fond enough of you to make sure that you were not left in a financially precarious situation after his death. Even if you were not, he was clearly aware of the clause in his insurance policy that excludes any payments to you or the children in the case of suicide. Or why else would he do his best to make this appear like murder? From the recent mentions of his disaffected former assistant and the ravaging of his office to this very, very clever concealment of how he managed to jump to his death, this wasn't just the work of a highly intelligent man, but also an act of kindness."

He turns to me then. "If I were you, I'd take a very careful look at his medical records. Any kind of fatal disease, or any incurable, mentally incapacitating condition would do as a motive." He jerks his chin at the at the stack of papers. "Considering that message, my money would be on the latter."

"What's the second fact?" the solicitor asks rather stupidly after a moment of dumbfounded silence.

"Oh, just look at it!" Sherlock Holmes exclaims impatiently, with a grand gesture at that fateful collection of documents. "He had style!"

And the widow breaks into tears.


"You look odd, Greg," MacDee remarks when I return to the office. "Anything wrong?"

I slump down in my chair. "Do you ever get the feeling, MacDee, that we've all missed our calling? All of us?"

He looks amused. "Not really. Well, one or two, maybe. Dimmock, definitely, now that you mention it." His friendly, open face looks ready to break into a full grin, but when he sees nothing of that kind on mine, he becomes serious again. "What is it?"

"It's just that I've always thought we were quite good at putting two and two together," I say. "But I've just seen someone solve a quadratic equation without a pocket calculator or even pen and paper, if you know what I mean. The kind with three unknowns, too."

MacDee raises his eyebrows, looking impressed. Then he grins after all. "Whoever he is, hire him."

And so it begins.