They have a marvellous time at the singing towers of Verilian. The suns graze the spires and River's heart soars with the music. She feels as though she could fly, like everything beautiful in the universe has been leading up to this point, when those high notes pierce her heart. She looks over at the Doctor to smile at him, and sees the tears on his cheeks.

She has never seen him cry before.

He doesn't talk about it all the way home, and won't let her bring it up either. He is silent, contemplative. Something is bothering him. River thinks it might be her absence.

"I'll only be gone a few weeks," she reminds him. "Just one archaeological expedition. A library planet completely abandoned. Doesn't that make you curious?"

She gets no response but a grunt from him.

"I'm surprised you haven't been there yourself," she continues, trying to see if she can get a rise out of him.

"I have," the Doctor says quietly. "Once. Long ago. Before I met you."

"What was it like?" River asks.

The Doctor gives her an imperceptable stare.

"Not somewhere I'd want to go again," he replies, effectively ending the subject.

When the TARDIS touches down at her house, River walks to the doors. The Doctor follows her.

"Have a good trip," he says, fidding uncomfortably with his bowtie. River thinks he looks unsually handsome tonight in his black tux and white cummerbund. Like the one he wore for Amy's wedding, all those years ago.

"I'll try to," River replies.

He reaches into a pocket - goodness knows this suit has dimensionally transcendental ones too - and pulls out a familiar object. The battered but still beloved sonic screwdriver.

"Here," he says, holding it out to her. "Take this with you."

River stares at him like he's grown an extra head to go along with his extra heart.

"Come again, Sweetie?"

"Take the screwdriver with you," he repeats. "It might come in handy."

"But... it's your screwdriver," River sputters. "You never even let me hold it."

"Well here's your chance."

River takes it reverently, cradling it in her hands.

"Are you sure?"

"Take it before I change my mind you daft woman," he snaps.

River laughs. That's more like the Doctor she knows.

"I'll return her in perfect order, I promise. Good as new. Better, if I can just tweak a few settings..."

"Don't you even dare," the Doctor says. And he's smiling now too.

River tucks the screwdriver in her pocket.

"I'll take good care of her," she promises.

The Doctor looks at her with such an intense gaze that River feels sure he is looking into her soul. She feels exposed. With her flimsy dress in the cool air, she shivers.

"You know... you know how I... Oh hang it all, River, you know I'm not very good at this," the Doctor stutters. "River, I..."

She knows what he's trying to say.

"I know," she says softly, taking his hands in hers. "It doesn't need saying."

"Sometimes it needs saying," he says. "I love you River, you know that, don't you?"

River knows. She stretches up on her toes and brushes her lips against his.

"I've always known," she replies.

The Doctor holds her tightly and kisses her till she has no breath left. He winds his hands in her sandy curls and doesn't let her go for a very long time.

"Be safe, my River," he whispers into her ear.

River touches his cheek.

"Be back for me in three weeks time, won't you? And don't keep me waiting this time."

The Doctor smiles.

"Goodbye," he says.

River doesn't look back, but she can hear the TARDIS disappearing. He's left the parking brake on.

He knew. River's last conscious thought as she slips away in that tomb of books is: he knew. It was the last time they would see each other, and he didn't tell her. She doesn't have it in her heart, in these last few seconds, to feel angry at him.

The Doctor walks into the TARDIS and closes the door. He pulls off his white bowtie and throws it forcefully at the controls. He is sending River to her death. She will step into that library, and she will never come back.

Another love gone. They all leave him in the end. Yet again he feels one heart and then the other break.

The TARDIS whistles sympathetically in his mind.

"I know," he says, putting a gentle hand on the console. "You're always here for me. I'm sorry I threw my bowtie at you."

He picks up the crumpled fabric and places it on the pilot seat. Reaching under the console, he opens a hidden hatch and pulls out the screwdriver. It's the one that River gave him, back when he didn't know who she was. It's how he 'saved' her, like a backup disk on a computer. Was that the best he could do? He gives a self-derisive snort.

Then, and idea comes to him, like lightening from the clear blue sky. It's so obvious. Why did he not think of it before?

But he did think of this before. He told her he would rewrite history. She told him not to. But when has he ever done what he is told?

The Library is quiet as the TARDIS lands. Now that he knows where to go, he parks close, so as to avoid the vashda narrada whenever possible. He shudders as he feels their presence. The empty, book-lined halls feel like a cemetary, each book a gravestone, marking a soul lost. The Doctor passes through them like a ghost. His Time Lord senses tell him that the shadow killers haven't detected his presence yet. He clutches the screwdriver so tightly in his hands that his knuckles turn white, and stays in the sunlight as much as he can.

The central processing room is as he remembers: cold and sterile and mechanical. He remembers the last time he was here, for Donna. Now he has come for River.

"Hello, CAL," he greets the computer. "I assume you know what I'm here for."

A light flashes on a console. The Doctor checks the readout. His younger self was here only a few minutes ago.

Inserting the sonic screwdriver, the one he's kept carefully for many years, into the processor is as easy as one of those twenty first century flash drives. Just plug and play, the Doctor thinks to himself. He types commands into the computer and watches as the bars of green light rise.

CAL beeps. It's done. She's all there.

"Maybe we could leave her less endearing traits in there?" The Doctor murmurs.

CAL blinks skeptically at him.

The Doctor smiles.

"Yeah I thought not."

CAL gives him the computer core equivalent of a withering look.

The Doctor unplugs the screwdriver, checking carefully to make sure he's got everything of her. Despite his joking, he doesn't want to leave anything of her behind.

"Thanks for this, CAL," he says, holding the screwdriver up. "Don't take this the wrong way, but I hope we never meet again."

The way back to the TARDIS is more parilous. The Doctor feels the vashda narrada just behind him at several moments, but manages to slip away. Once, he almost loses hold of the screwdriver, in his mind's eye seeing it fall from his numb fingers and slip away into the darkness where he can't follow. But he catches it with his other hand, and slips it into one of his bottomless pockets for safe keeping.

He doesn't think he's ever been so glad for the sight of his familiar blue box, standing in a puddle of sunlight from an overhead window.

"Hello beautiful," he mutters as he slips inside.

He climbs the ramp to the control console in three large bounds, and fumbles the screwdriver out of his pocket. He prays to whatever cosmic force is listening that this will work as his shaking hands plug the screwdriver into the TARDIS mainframe.

A light flickers.

And then she's standing before him, beautiful, impossible River, wearing a flowing blue gown the colour of the TARDIS, her sandy curls flowing down her back. She turns around to face him.

"Doctor?"

He grins widely.

"Hello again, River."

River takes a step forward and puts a hand on his arm. It passes right through. A look of shock and horror crosses her face.

"Oh. I can fix that," the Doctor says quickly. He types on the typewriter, and her image solifies. He reaches out and takes her hand. "There. Better?"

River flings herself into his arms, hugging him tightly. When she pulls away, her face is confused.

"I don't understand," she says. "I just saw you. Younger you. I died. You uploaded me into the library mainframe."

The Doctor nods.

"And now I've uploaded you into the TARDIS. It hit me like a ton of bricks just now: why let you stay there when you could stay here?"

River moves to the console, staring at the sonic screwdriver, which is still plugged into the TARDIS.

"I'm hooked up to the TARDIS?" she asks.

The Doctor nods.

"Right now you exist inside the TARDIS mainframe. She's projecting an image of you for me to talk to."

River tilts her head, as if listening to something.

"She says I'm projecting an image of myself," River clarifies. "I'm a separate entity than her, sharing a place in her matrix." River laughs. "We're flatmates."

The Doctor takes both River's hands in his.

"It was the only way to save you," he says. "I'm sorry."

"For what?" laughs River. "We're together again and I'm not dead. I call that coming out on the winning side."

"You'll never be able to leave the TARDIS," the Doctor says regretfully. "You're basically a hologram. I've doomed you to a life trapped here."

"The TARDIS says that's not strictly true," River replies. "She says that if we can find a way to expand her range of transmitting, she can project me outside. Like she does when she translates for companions."

"I'm not sure I like this whole you talking to her thing," the Doctor mutters. "I have a feeling you two are going to gang up on me."

River grins.

"You have no idea." She stretches on her toes and kisses him. Automatically his arms come around her, large fingers playing with the soft blue fabric of her dress, fingering the strap over her shoulder. "At least you know I won't leave you," she says, leaning her forehead against his. "My life is as long as yours now."

"I don't know whether that's an advantage or not," the Doctor replies, grinning cheekily.

River hits his shoulder.

"Watch it. I can control the atmospheric conditions now."

The Doctor's grin stretches wider, and he bounded around the console.

"Let's get out of here. Let's go have an adventure, River, just you and me."

He punches in random coordinates and pulls the lever. Lights flash as the main stem of the console pulses to life. The familiar sound of the TARDIS groaning to life comes on as she throws herself into the void.

"You left the parking brake on again." River tilts her head. "There. I fixed it for you."