This takes place between Dinosaurs on a Spaceship and A Town Called Mercy. It's inspired by something Steven Moffat said in an interview about River Song's squareness gun.

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"Where are Amy and Rory, then?"

The Doctor jumps at the sound of the voice from behind him – even though it's familiar and not entirely unexpected, he's still startled. He flips a switch on the TARDIS's console to keep her securely in deep space so she doesn't accidentally collide with ancient Greece (again) and turns around to see the owner of the voice – tall and slim, wearing her favorite denim jacket over a crisp white shirt, blonde hair as wild and curly as ever, a small smirk on her face.

"They're at home," he replies, coming down the stairs to greet her. "They asked me to drop them off."

"What sort of trouble have you three been getting into?" River Song asks.

"Well, most recently," he tells her, "dinosaurs on a spaceship."

"That's wonderful, sweetie."

"I thought so, too," he agrees. "How did you get in here?"

She sighs and rolls her eyes. "Really, sweetie, are you ever going to stop asking that? I did. Thought you might like this." She reaches into the pocket of her jacket, pulls out something small and metal, and tosses it to him. He catches it with ease, and once it's in his hands, he registers what it is: a dull silver ring.

"Just to make it official," River reasons, and again reaches into her pocket. "I took the liberty of picking one out for myself as well, I hope you don't mind," she says as she pulls out a small silver band adorned by a single princess-cut diamond and slides it onto her ring finger. After a moment, she stops, and give him a questioning, almost worried look. "We have done that, yes?"

"Yeah," he confirms as he tucks the ring into the pocket of his tweed jacket.

"Good," she says. "Otherwise –" She laughs lightly. "Spoilers. Can't have that." She pulls off her denim jacket and drapes it over the railing; as she does, his eyes are immediately drawn to something on her hip, and he points to it, saying, "River, you brought your gun on my TARDIS. No guns on my TARDIS!"

"Sweetie –"

"Don't 'sweetie' me, you remember what happened the last time you brought your gun on my TARDIS?"

River rolls her eyes. "I was brainwashed, remember? Don't worry, I'm not going to shoot her again."

"Still," he insists, holding out his hand and wiggling his fingers. "Hand it over." River sighs and rolls her eyes again, but she consents, taking her gun out of his holster and placing it in his hand. "I'm confiscating this," he tells her, holding the thing between his pointer finger and his thumb and carrying it up the stairs, placing it delicately on the TARDIS console. "You can have it back when you leave."

River sighs, but doesn't protest. "So. Where are we off to?"

"Well, I've got a few ideas," he says. "I was going to take Amy and Rory to ancient Egypt next trip. We could go visit Sarah Jane, she can always use help defending the Earth. Or you could just flip through the TARDIS's files, see if anything strikes your fancy."

"Alright then." She hurries up the stairs to his side, grabs the screen, and pulls it around to her. As he picks her jacket up from the railing and hangs it from the coat rack, she begins to go through the database; after a few seconds, she calls out, "Barcelona's got dogs with no noses."

"I know," he calls back. "How d'you think they smell?"

She rolls her eyes. "You think you're so clever," she mutters. "So, shall we go, then?"

He pauses. I was gonna take you to so many places. Not the city Barcelona, the planet Barcelona. Imagine how many times you tell that joke every day and it's still funny! Cancel Barcelona.

"No," he says. He can't dwell in the past, shouldn't dwell in the past – he's married, for crying out loud, and by this time, she probably is as well – but if he meant to take her there and never did, he's not taking River either. He's sentimental like that.

She gives him a look, but still, she shakes her head slowly and continues looking through the TARDIS data files. "Woman Wept, maybe? No, never mind, I've been there, it's rubbish. Appalapachia?"

"No!" he exclaims insistently. "No, no, definitely not. Chose somewhere else."

She raises an eyebrow. "Well, alright then." She continues searching the databanks, murmuring to herself, "No… no… been there… no… no… no… hey!" she exclaims finally, looking at him with a bright, eager expression on her face. "The Singing Towers of Derilium! Oh, I've always wanted to go!"

You took me to Derilium to see the Singing Towers. He looks down at the floor, unable or unwilling to reply, he doesn't know which. The Towers sang and you cried. Slowly, he turns his face up again, forces himself to meet her eyes; by her expression, he can tell that she's realized something's wrong. I suppose you knew it was time.

No. It's not time yet. Not yet.

He can't stop this from happening – it's fixed. He himself witnessed it. It has to happen. He can't prevent it.

But dammit, he can postpone it.

"Doctor?" she asks hesitantly, clearly demonstrating that she understands something's not right by calling him by his name instead of simply 'sweetie'.

"Not there," he says softly, and then forces his features into a smile. "I'll take you there someday, River Song. Just not today."

She nods. "Alright. Where, then?"

He pauses, and then goes to join her at the screen. "You know what? Let me pick. You go put on something nice."

She glances down at her clothing. "What's wrong with this?"

"Something formal, River," he sighs. "Maybe that green dress –"

"No," she interrupts, cutting him off. "Don't even start. I'll pick something, thanks." And she turns and heads off to the wardrobe.

He takes his time programming in the coordinates of 19th century Paris. They've never really done this, a proper date, so to speak. It's always aliens and danger and running for their lives. Knowing him, this will probably work out to involve a lot of those three things as well. But he's going to try his best to turn up on a day that aliens don't invade.

"Sweetie?" It's only a few minutes later that River comes climbing back up the stairs, a long-sleeved black dress with a low V-neck clinging to her curves, a sort of military cap perched jauntily over her unkempt curls, and something dangling from her left hand. "What were you saying about guns on your TARDIS?"

"I said no," he tells her simply.

"And what about this?" She holds out the object in her hand, and he looks at it, really looks at it – and he recognizes it.

"Oh, that's not a gun," he says, taking it from her and turning it over in his hands. "Not a weapon. It's a sort of sonic blaster. It's called a squareness gun." He frowns at her. "Where did you find it?"

"In the wardrobe," she replies. "With this." She takes off the hat and tosses it to him; he catches it, and quickly realizes that he recognizes it as well. It's military, World War Two, circa the 1940s. Well, 1941, to be precise. "So where did they come from?" River asks. "Never figured you one to wear a military cap."

"I'm not," he replies, throwing it back to her. "It belonged to an old friend of mine. Long, long time ago. He left it here, before he –" The Doctor stops. Before he what? Left? Got left behind? Died? All of the above?

After a moment, River ventures, "Did he die?"

"Oh, yeah," he replies. "Loads of times. He's in 21st century Cardiff now, got a pretty good life."

River either hides her confusion very well or has known him for long enough that nothing can surprise her now; at any rate, she takes the squareness gun back from him and asks, "Can I try it out?"

"I'd rather you –" he begins, but before he can get to 'didn't', she's whirled around and is firing the gun. It begins to emit its trademark glowing blue squares, expanding outward until they reach the wall; with a brief shimmer, a large square of the wall vanishes, revealing the corridor beyond it.

"Oh, that's magnificent!" River exclaims gleefully. "Oh, I am keeping this."

"Um, River, if you don't mind," he says, "could you put the wall back?"

"Oh! Right." She looks over the gun, searching for the right setting. "How do I –"

"Use the digital rewind," he says, pointing. "It's right there –"

"Right," she agrees, and his the digital rewind button; the missing square of the wall reappears. Grinning happily, River tucks the squareness gun down the front of her dress (where it is surprisingly well concealed) and glances over at him. "So, have we landed, then?" When he nods, she asks, "Where are we?"

He doesn't reply, simply gestures to the door, silently telling her, go and see.

She smiles brilliantly and turns, running down the stairs and throwing the door open. Unknown to her, as she rushes out of the TARDIS, he turns away from her and leans heavily on the console, his head down.

She wants to see Derilium.

She has a squareness gun.

When he woke up this morning, he didn't know he'd be coming two steps closer to the day when he'll finally have to say goodbye to her.

So he takes the silver ring she gave him earlier out of his pocket and holds it up to the light, carefully inspecting it before sliding it onto his finger. Might as well make the most of this while it last. He can't have much longer with her.

But he's not going to dwell on that now. He's going to go out there and have a lovely dinner on the banks of the river Seine with his beautiful wife.

So he turns to the door, steels himself, and walks down the stairs, stubbornly banishing all thoughts of Captains, Songs, and squareness guns.