John's dreams are always of battle, when he dreams at all. Usually the dreams are of heat and sand, but sometimes he dreams of a larger world, of a gun far too large for his own small hands, and of grinning to one side at a girl he called sister.

He never expected to see her again in his waking hours. Not after he'd escaped Manticore; not after he'd left the country; and certainly not after the Pulse.

And yet, here she was.

Sherlock glanced boredly at the view from the air traffic control tower at Mycroft's private airstrip. "She's had an unpleasant flight. And apparently, Mycroft has lost her luggage."

"How do you know she had any to begin with?" John asked, silly smile not leaving his face.

"Well, she's clearly agitated, but more the way she's dressed. Those are nice clothes. They are likely, therefore, not to be her only clothes. And yet no one is making a move to bring her a suitcase. I would have thought…"

John's awareness of the conversation drifted to the back of his mind as a scent came to his attention, low and musky-sweet, like one of those old gin and tonics Dad used to drink on Fridays, when Mum put the Bing Crosby record on to play. But this was rougher than the slow dancing he'd caught them at from time to time – this jungle scent carried with it memories of a deep, deep need and sweaty moaning, of hands kneading-

"John?" said Sherlock, touching the X5's shoulder. John stiffened and closed his eyes, bringing himself under control. "Is it the serotonin?"

"No," John said. "There's this… this scent…"

Sherlock frowned. "Tell me."

"It's… god, I don't know. It's like nothing I ever smelled before, and it's making me want to… fuck!"

Sherlock blinked.

"No- well, yeah, it's making me want to have sex with whoever's putting that scent out." John put his hands over his eyes. "I don't know what's happening to me."

Sherlock made a soft sound of understanding. "She's in heat," he said.

John looked at him in surprise.

"It was in the files. Due to your felinoid DNA, female X5s go into heat three times a year, experiencing an intense sexual desire. There was a footnote that any other X5 units would likely be out of commission for the duration. I should have realized why."

The head-scent was drawing nearer – John shuddered. It was his sister – he never wanted to- but now he desperately wanted to-

"It only lasts approximately ten to fourteen hours," Sherlock said. "If you wish me to give you some privacy-"

"God, no, Sherlock!" John said. "It would be like you having sex with Mycroft! She's my sister!"

Sherlock nodded, and picked up his phone and dialled. "Mycroft. She's in heat. It's affecting John." A pause. "The second one. Good." He slipped the phone back into his pocket. "She'll come around to the flat once it's passed and she's had the chance to sleep and shower."

John nodded, and started to relax as the scent faded. Then, he tensed again. "Oh, no."

"It hasn't stopped?"

John shook his head, trying to clear it as much as he could. It wasn't quite so bad as before – but still… "Ten to fourteen hours?" he asked. He could last that long. As long as Sherlock didn't-

Sherlock took off his jacket and his shirt, kneeling down carefully in front of John. "Yes," he said.