This fic is AU, assuming that the Master did take up the Doctor's offer and didn't get shot. It is also not meant to be taken seriously. The BBC owns the canon; I merely own this particular scenario.
"Why me?" Jack spluttered. Of course he had run to the TARDIS when it showed up on a sunny Saturday morning in May, but the Doctor had to ruin it by making an unreasonable request the moment he stepped out and quickly shut the door behind him. "Why does it have to be me? You can refuel in Cardiff at any time in its history."
The Doctor gazed at him with puppy eyes. "I want to surprise Martha and be at her graduation from medical school. When I need to leave the TARDIS on other planets or in other times I can put those cuffs on him and keep him within ten feet, but people know his face here. He's supposed to be dead. That could cause all sorts of problems."
"Just lock him in the TARDIS then."
"Last time I tried that I came back and he'd taken a bite out of every single one of my pastries, replaced my toothpaste with hair conditioner, constructed a death ray out of spare junk, and drawn little mustaches on the characters in all my graphic novels."
Suppressing a laugh, Jack asked neutrally as possible, "You have comic books?"
"The Sandman is not a comic book series; it's groundbreaking literature. In any case, you're my only friend who both knows how dangerous he is and could actually pose a threat to him. He's shackled to a chair with some books and an iPod Touch to amuse himself with. All you need to do is keep an eye on him. Read a book or something. You can even gag him if he gets annoying."
"Uh, how did you get him to agree to be shackled to the chair?"
"That's neither here nor there," the Doctor replied, far too quickly.
"What if he needs to go to the bathroom?"
"Time Lords can go several days before needing it. He just went."
"That is so not fair."
The Doctor smiled. "I'll give you his screwdriver, set to 'stun'. That's if he somehow gets out. I don't want you making him regenerate, though."
"Now I get why you want me to do this. What I don't get is why I want to do this."
"He can't hurt you anymore." He awkwardly grasped Jack's arm as if that would make him more convincing. Or maybe it was acknowledging Jack's past suffering but pleading with him to look beyond it.
It was a little harder for Jack to argue when the Doctor was touching him; he'd give him that. "That doesn't mean I want to spend a day in his company."
The Doctor grinned in his daft, entirely-too-adorable-in-this-regeneration way, and withdrew his hand, instead sinking it into a coat pocket. "Well, maybe this would convince you." After some rummaging he produced an octagonal fruit about the size of a bowling ball.
Jack squealed, "It's a Star Melon! You have a Star Melon!" One of his recurring dreams was his mother cutting one open for him, and waking up just before he was going to have his first bite. He didn't even remember what they tasted like anymore.
"You told me once that was your favorite fruit. Now that the TARDIS doesn't fly properly when you're on board, you won't get to have one until the 48th century at least. Or when you get your hands on a different, reliable time machine that isn't allergic to you." He found a spoon in another pocket and waved it at him. "What do you say?"
"Okay. Okay. You win. But I should be allowed to punch him if he deserves it."
"Just don't break anything." The Doctor handed him the Star Melon and the spoon, then gave him a hug. "Thank you, Jack. I won't forget this."
"Rassilon, Jack Harkness whoring himself so he can eat a melon. I don't know if there's even any room to add a double entendre."
The Master was strapped to a comfortable lounging-style chair, with his feet up, by the ankles, waist, and upper arms. His lower arms and hands were free so he could hold a book or play a game on the iPod. He'd gotten addicted to the early 1990's SimCity recently – there was something so satisfying (though a pale imitation of the real thing) about tenderly growing a thriving metropolis and then unleashing Godzilla upon it. And setting it on fire. Maybe a few tornadoes if the populace got too uppity.
All in all, the Doctor had been extremely kind. The Master was still incensed to be babysat by the Freak and was mentally working out a plan of revenge.
Jack just beamed at him, haven taken his first bite of heavenly, long-forgotten goodness. "If you want me to flick seeds and pulp at you, I am completely willing to do that. And it's not whoring. You're not that lucky."
"You're not full of yourself at all," the Master sneered, rolling his eyes. "And I know you've never shagged the Doctor."
"I don't mind. It could jeopardize our friendship."
The Master noticed a little twitch of Jack's eyelid when he said that. The drums got louder. "No you're not. You want him. You've pined for him for more than a century."
"I do have permission to shut you up, you know."
What good does that do if I can talk telepathically?
Jack smirked. "I could always knock you unconscious. That would be fun."
"You don't want to do that."
"How do you know?"
The Master licked his lips and locked eyes with Jack. "Because I have slept with the Doctor, and I am about to tell you every one of his embarrassing secrets."
Jack swallowed another mouthful and tried not to look too glazed. "Huh."
"Are you stunned by my offer or by the fact that the Doctor is getting cuddly with the person who nearly destroyed your planet?"
"If the only other human left in the universe was interested in me, it wouldn't matter what he or she had been doing. I'd need it." A long pause. Then Jack leaned closer and whispered, "So, what's he into?"
The Doctor skipped his way back to the TARDIS humming "Pomp and Circumstance". It had been good to see Martha having a real life of her own, one not tied to him, one where she could help others but not be in constant danger. And her family was doing well.
The only thing he worried about was what kind of scene would greet him at home. He'd expected Jack to call and complain, but there had been no calls whatsoever. This either meant peace or trouble he hadn't suspected. Peace was unlikely.
Opening the door and bracing himself for whatever domestic horror could await him, he heard peals of laughter. The surprising thing was that it came from both the Master and Jack.
Jack was choking out, "The thing with…the thing with the riding crop…I've had something like that happen…you just need to have some antiseptic on hand…but when you add bananas…"
"Don't forget the corset!"
"Stop it! Shit, I'm going to die here…best death I've had…and…and…and a set of bagpipes…"
The Master could barely talk between wheezes. "It wasn't only that…it turned out Frisbees are…you know…like I said before…but the paint was too hot…and…."
"Time Lord physiology is…interesting…"
The Doctor cleared his throat. Jack was too busy rolling on the floor trying to hold his sides together to see him. The Master looked very smug. "I told you when you tricked me that there would be hell to pay."
"What did you tell him?!" The Doctor felt very hot. Human temperature, even. His blush must have been something to behold.
Jack tried to calm down but his body shook with repressed giggles. "Doctor…you know I love you and respect you and everything…and I'm cool with all the fetishes that don't hurt bystanders…but playing croquet in the shower?"
"He knows everything. You tie me up and leave me, I kiss and tell."
For a terrible moment the Doctor felt defeated. Then with grim determination – and an ounce of madcap glee - he took off his tie.
"Jack, he just used you for payback. Mind if I follow suit?"
Jack sat up with an eyebrow raised and mischief written all over his face. "Will I enjoy it as much as this?"
The Doctor started unbuttoning his shirt and winked at Jack. "Most likely."
"Oh no. No. Don't you dare," the Master hissed.
"I believe this will make us even." He descended upon Jack with a ferocious snog. The Master ranted in what Jack assumed was obscene Gallifreyan. The TARDIS never translated swearing, old-fashioned girl that she was.
The Doctor got up, though Jack whined at the loss of contact, and kissed the Master to quiet him. "Be good and I'll let you join in."
The Master stared blankly at the Doctor. He looked at the now-naked and very mussed-up, and even more pleased Jack. Then he said: "Promise?"
The next time he came up for air, Jack gasped, "This is the best day I've had in a long time."