When he gets the message from River's university, it's only been six months since the relationship officially started. They met years ago, but everything up until then has been a jumbled mess of crossing timestreams and mistaken identities. Finally moving along the same timeline, at least for half a year, has been absolutely wonderful, full of adventures and friendship that eventually developed into what was destined for them.
They've been traveling together in the TARDIS, just the two of them, for two months now. River had referred to it as their honeymoon, the Doctor had referred to it as damage control - the TARDIS could be a bit of a jealous girl sometimes.
River is sleeping when the transmission came through, and since there are no more secrets or spoilers between them, he opens it and listens.
He hadn't expected this. Well, he had, but certainly not so quickly. He sits down on the bench as the horrible memories of the Library came crashing down on him. The message had grandiously declared that Professor River Song had been accepted onto the team being send on the Library expedition. Their ship would leave this weekend, only days away.
It wasn't fair, the Doctor thinks angrily, thinking of River still asleep in the bedroom, unaware of the danger awaiting her. She's so happy. He's so happy, but at least he knows it would only be temporary. River is planning on so much more - travelling with him, having her own adventures, maybe even having a family (although he wasn't sure about that just yet). None of that will ever come to pass, and the fact that he's known it all along is the only dark cloud hanging over their relationship.
He programs some coordinates into the TARDIS's flight path, throws a lever, and they lurch into orbit toward a destination for the first and last time.
He wanders through the halls of the TARDIS until he reaches the bedroom. As he quietly eases open the door, he sees her still in bed, eyes closed. He's about to leave when she opens her eyes and smiles sleepily at him.
"You're up early. Did you even sleep last night?" she asks, propping her head up on one hand, her curls spilling onto her shoulder.
He walks over to the bed and perches on the edge, watching her. "Someone kept me up most of the night." She laughs, grinning cheekily at him.
"Didn't hear you complaining at the time, sweetie." She takes his hand and starts to tug him toward her. "Where are we headed?"
He lets her pull him down on top of her for a quick morning kiss. "Somewhere important. Big day today. Well, every day is important, every day is big. But this is something you can't miss."
River sits up in bed, curious. "Where are we going?"
He taps her on the nose and hops off the bed, turning away so she can't see the pain in his eyes.
"New suit, new haircut... taking me on an actual date where we don't end up in mortal danger," River says with a grin. "I'd think you were proposing again."
"Still have the bruises from the first time," he replies, and she laughs, inching closer to him as another gust of wind blows past them. His arm slides around her waist, fitting naturally into the curve of her back. Their vantage point is a cliff overlooking the towers below. As the wind blows through them, the most beautiful songs echo around them. He's read about them, read about how they work, but he's never visited before.
"You're awfully quiet today, sweetie," she tells him. "It's a nice change, don't get me wrong, but I do worry about you when you get like this." River turns toward him, concern on her face. "What's going on?"
"I don't mean to pry, but the TARDIS intercepted a message meant for you. Something about an expedition to a place called the Library-"
River's eyes light up, the smile on her face so beautiful and brilliant. "Oh! I've been waiting to hear back. Did they accept my application? They did, didn't they?" She leans her head against the Doctor's shoulder and sighs happily. "Honestly, the wait was killing me. Once-in-a-lifetime-opportunity, that. Nobody's been there in years."
"Were you going to tell me? That you were going?" the Doctor asks, trying to keep the emotions out of his voice. "Or were you just going to run off again?"
River lifts her head up off of his shoulder and narrows her eyes at him. "Sweetie, we've been over this. You knew what you were getting into - you married an archaeologist, after all." She smiles and then affectionately pats his cheek. "I'll be gone a few weeks, maybe a month. I'm not sure what they're expecting to find over there, but you know I love a good mystery."
His arm tightens reflexively around her waist. "Need any help? I could... come along, if you wanted."
The answer is exactly what he was expecting. "I'll call you if I need you, as always."
They lapse into quiet, listening to the wind shifting amongst the towers, the beautiful melody twisting and turning like leaves in the breeze. River shivers slightly in his arms, moving closer to him so that she's in his lap, her one heart beating against his two.
When the silence is broken, it's because he knows the song is about to end and he doesn't want to hear it. Fumbling in his jacket pocket with his free hand, he searches amongst odds and ends - psychic paper, a good amount of lint, some rogue jelly babies, and then finally he finds what he's looking for.
"So, Professor Song, if you're going to this Library without me, you might need a bit of help." She raises an eyebrow again, about to protest, but he forges onward. "Did some modifications, so it's got some extra features. It's not the prettiest - sorry about that - but it works." He hands her the sonic screwdriver, watches her expression change from one of suspicion to one of wonder and awe.
"You're giving me your screwdriver?"
He smiles slightly, enjoying her surprise. "Well, I've got a new one now. Green. I always preferred the blue - reminds me of the TARDIS."
River hugs him suddenly, so tightly, and as he tries to free himself from the pile of curls now brushing against his face, he hears her whisper "thank you" in his ear. They stay like that, together, for a long time, until the final strains of the towers' song has faded away into the window. When she finally pulls away, her eyes meet his, and they're wet.
"You're crying," she whispers, reaching out to wipe away his tears. "Doctor, what's wrong?"
He shakes his head, swallowing to try and dislodge the lump in his throat. And the answer comes easily, and so honestly. "Nothing. For once... everything's just right." The only problem is, he thinks as River leans in and kisses him, her hands in his hair and his wrapping around her back to hold her close, is that it's now over. Everything River told him the first time they met has come to pass, the most wonderful night of his life playing through as if scripted.
They part for a moment, her grey eyes shining with tears of her own now. Wordlessly, they rise from their blanket, packing up and linking hands afterwards before walking back to the TARDIS. As he unlocks the door and lets her in, he takes one last look back at the singing towers - silent, dark, lonely. It will be thousands of years before they sing again, and he will never return again. It would break his hearts.
River's hand reaches for his again, pulling him into the TARDIS as she smiles at him. "Hurry up, I need to start packing. You can help."
In the end, they don't get much packing done. When he wakes up the next morning, she's already gone - the only clothes on the floor of the bedroom are his.