River stared at the TARDIS as it disappeared, her heart aching. The Doctor didn't realize what that meant – from his point of view, it was their first kiss. He couldn't know that that meant it was her last, it wouldn't occur to him.
She knew that this meant it was the last of the good times, that now she was left with "Hello, Sweetie" 's and maybe a shared smile or joke if she was lucky. One day soon, he wouldn't know her. And she would see him, not her Doctor anymore. She had told her father earlier that it would break her heart, and it would. She had seen the Doctor's face sometimes, when she asked about the first time he met her, how it would crumble, just a little, and soon he would reach for her hand, her hair, evidence to prove she was there, that she was okay. And she knew that it wouldn't be a goodbye like in the stories, where she could kiss him or laugh about the things they had done. He wouldn't know her, she couldn't kiss him, and they couldn't talk without spoilers on either side.
River hoped that she was wrong. Maybe on those days the Doctor had remembered a past companion, a past life, a past loneliness, and just needed reassurance. Maybe the TARDIS did jolt a little, making him catch his balance. God knows he's clumsy.
The TARDIS finally flashes completely out of view, last echoes of sound disappearing. She turns back to her cell, hands braced on the bars, and reaches for the button to call security so they can lock her back in. She thinks she really should have a key by now. Surely they realize that there's no way to keep her in when she wants out, and it's a waste of their time to come and lock her back in.
Before her hand reaches the button, faint beeping and whirring comes from behind her, and she spins towards it as it grows louder.
The TARDIS reappears, same spot as before. Confusion makes her frown, at least until the Doctor exits.
It's her Doctor. He jumps out of the TARDIS and stares at her, smiling. "Hello, sweetie."
She barely has time to tell him that That's my line, thank you very much before they're kissing again.
When they break apart, foreheads still touching, he looks into her eyes. "It wasn't fair, River, to leave that as your last kiss. I knew when it was, logged in the coordinates… and I came back. I'll come back again, I've lived through this time, I know when you're alone and my past self can't know you."
She laughs giddily, relief loosening the tension in her muscles. Her head drops to his shoulder, and they wrap their arms around each other. "Thank you. Just… thank you."
His hearts beat in tandem with her own, a soothing rhythm as they stand there, in Stormcage prison. His chest vibrates as he speaks. "We should get some linear time. Timey wimey stuff can only go so far. Time isn't the boss of me." He smirks, and she realizes that he must say this sometime in his past and her future. Reusing lines, is he?
She soaks up his warmth for a few more moments before pushing him back towards his TARDIS. Her fist hits the security button, and the alarm wails out again. As it wails, footsteps begin to pound in the distance. She can almost hear their grumbling, Oh look, Doctor Song is back.
"You better go before they shove you in a cell too, for breaking in."
They kiss one last time, before the footsteps get alarmingly close and she opens the TARDIS's door for him. "See you, sweetie."
He grins at her, and the door closes, the TARDIS whooshing away just in time for the armed guards to round the corner and point their guns at her. She steps back into her cell without a complaint.
And waits.
