He's leaning in to kiss River when the door opens and a woman walks into the TARDIS.
“Oops,” says the woman, “my mistake, pretend I wasn't here.”
“Oh, my,” says River, eyes widening.
“Wait,” says the Doctor, “do I know you?”
“Sort of,” says the woman, standing by the outer doors.
River walks over to the new arrival, looking her up and down with an appraising gaze. “Oh, that's nice,” she says. “That's very nice. Well done, Sweetie, have you had it long?”
The Doctor frowns. “River! You're only supposed to call me Sweetie!” He stomps over to the women, scowling.
“Am I indeed?” says River with a quirk of an eyebrow.
“Yes! It's our thing!” He looks at the intruder. “I do know you, don't I?”
“Was he always this slow?” asks the woman.
“Adorable, isn't it?” says River with a fond smile.
The Doctor looks at the woman carefully. He walks around her. He sniffs at her hair. He's about to poke her in the chest when she grabs a hold of his wrist.
“Oi,” she says, “don't be rude.”
“Wait,” says the Doctor.
“Oh,” says River, “here it comes. Penny in the air...”
“You're me!” he cries.
“And the penny drops!” River touches his arm affectionately. “I knew you'd get there in the end.”
“You're a girl,” says the Doctor, observant as ever.
She looks down at herself. “Am I? I hadn't noticed.”
“Don't be sarcastic,” he says. “Anyway, why are you here? You know we're not supposed to cross our own timeline without a very good reason.”
“I came here specifically to annoy you,” she says. She sighs. “I'm parked around the corner, mistook your TARDIS for mine.”
River interjects. “As entertaining as it is to watch you argue with yourself, I do have a better suggestion.”
“What?” ask the Doctors simultaneously.
River smiles her special smile. “We should get to know one another a bit better. Why don't we see if there's a big enough bed on the TARDIS?”
The Doctors look at each other. They look at River. They look at each other again.
“Oh, go on,” says River, “how often do I get the chance to have a threesome with my own husband? Or wife, as the case may be.”
“River, don't be weird,” says the earlier Doctor.
“How is it weird? Don't you think she's pretty?”
He looks at his future self again. “I think Suit-and-Sideburns was prettier.”
“Oh, he was,” agrees the other Doctor.
The Doctor nods. “Everyone fancied me back then, it was flattering but a bit awkward at times.”
“I did like Reinette,” the other Doctor says with a wistful smile.
“Oh, yeah, she was good.”
“And Mickey, if he wasn't already married.”
He looks at her. “Mickey? Seriously?”
She shrugs. “Why not?”
He thinks about this for a moment, tilting his head to one side as he contemplates. “He did have a certain boyish charm about him, I suppose.”
“Are you two going to spend all day daydreaming about your previous conquests?” asks River.
“It's not like I've been carving notches in the bedposts,” he protests, “I'm just reminiscing with an old friend.”
“Oh, so you're friends now, are you?”
“She's growing on me,” he says, non-committal. An idea strikes him. “Oh! Shall we compare sonics?” He reaches into his pocket for the screwdriver.
“I'm fairly sure your dick is bigger than hers,” says River with a roll of her eyes.
“River! It's not about... that! I just want to see what improvements I'm going to make to it in the future.”
The other Doctor produces hers from a pocket. “Okay, check this,” she says. She presses a button, music fills the room. “I've got all my iTunes playlists on here.”
The Doctor is impressed. “Living in the future!”
“And,” she continues, “I can use it to summon the TARDIS from fifteen light years away.”
He raises his eyebrows. “How did you manage that?”
“Spoilers,” she says, “but I did have some help.” She pockets the device with a cheeky grin.
“Have I been ginger yet?” he asks, ever-hopeful.
“Not yet, but you can look forward to some amazing eyebrows.”
“How amazing?” asks the Doctor, aware that his current eyebrows lack a certain something.
“They're worshipped as a god on at least three planets.”
“I'm fairly sure all this counts as spoilers,” says River.
“You never let me have any fun,” he grumbles.
“Yes, she does,” says the other Doctor with a saucy wink to their wife.
“Okay, but apart from that.” He nods to his other self. “I suppose you should go before we accidentally break the universe somehow.”
“Better safe than sorry,” she agrees. She looks at River, and then at the Doctor. “Do you mind if I kiss my wife before I go?”
“Oh, please do,” says River with a smile. She steps towards the Doctor from the future and puts her hands on her waist as they kiss. It goes on for some time.
“I don't want to be rude,” says the Doctor, “but are you two going to be much longer? Only I could clean out the blue stabilisers if you want some more privacy.”
“No, I'm good,” says the Doctor, moving away from River and licking her own lips. She nods to the Doctor. “Be you later,” she says, and then she's gone.
“I like her,” says River, when they're alone again.
“More than you like me?” he asks, worried.
“She's you! How can I like you more than I like you?” She touches his chest. “Honestly, Sweetie, you're ridiculous sometimes.”
He presses a light kiss to her lips. “Sorry,” he says, “I'd never make you choose between myself and me.”
“Good,” she says. “So, can I spend some quality time with your penis while you still have one?”
He blushes at that, but he manages to say, “That's not a bad idea.”
River takes his hand and he follows her out of the console room.