Inspired by the mini episode Last Night, but understable without seeing it.
He'll take River all the way back to Stormcage after the Singing Towers, but he'll still go through the motions of fixing that vortex manipulator one last time. He'll materialize the TARDIS inside her cell, making up excuses to delay her departure. Before taking off, he'll kiss her one last time—last but not last, they always met in the wrong order—running his fingers through her knotted hair and rejoicing in the warmth of her cheek against his.
Only when she tries to break away will he become awkward again, hands awkwardly waving in the air, nearly tumbling backwards as he tries to leave. River will tease, and the word goodbye will dribble off his tongue, wanting to drag the word forever along with it. But he'll stop himself, without using the word spoilers because that word will hurt for a very long time.
When he shuts the door behind him, he will sink to the floor and bury his face in his hands. He won't cry because he'll wake up his companion—of course he'll have a companion, he couldn't bear to be alone after River's last night with him.
Something, maybe a bad dream, maybe a whisper from the TARDIS, will wake the companion up. He or she will creep down to the control room, see the Doctor's face and ask what's wrong.
He'll try to laugh it off, maybe suggest a trip to the Eye of Orion or Barcelona, but the TARDIS will not let him escape so easily. It will land in the backyard of a sleepy house in Leadsworth, the home of Amy and Rory Williams.
He won't step out right away, but he'll do it before the TARDIS kicks him out. He'll straighten his bowtie and pause before knocking on the door. His mind will flicker to boys who delivered black-edged telegrams in the second before Rory opens the door.
Amy will scream
Amy will cry.
Amy will hit him.
Amy will refuse to believe it, that much he knows without any foresight.
Rory will hold back his own reaction, choking on tears as he tells the Doctor thank you for the news, and couldn't he share a few more detail? Their waiting will be over, at least until she breaks out of Stormcage for the fifth time instead of the final.
He'll offer them one last trip, but he knows they will refuse, because the only trip Amy would want would be one to stop River from ever going to the Library.
Time can't be rewritten.
Not these times.
Not one line.