The Doctor was sitting on River's TARDIS-blue couch waiting for her to finish changing into a nice outfit. He looked around at all the perfectly normal furniture and paintings with a few odd trinkets from other planets scattered throughout. The Doctor recalled which date each object was associated with and sighed. He thought that he might start taking her out more often. It must drive her nuts sitting around in a normal house grading papers. Perhaps that was why she was always so excited for their adventures.
River returned in a stunning green evening gown with a cup of tea for each of them. "How did you know it was me?" the Doctor asked, taking a sip.
"I always know," she responded. "I especially like the haircut."
"So you don't mind, then?" he continued. "That I've changed."
"Everything changes and has its time," River replied, "but my love for you is the one exception."
The Doctor grinned. He knew he could always count on her to make him feel better.
"So where are we headed tonight?" she inquired.
"Wherever your heart desires," the Doctor said.
"Hmm…well I've always wanted to see the singing towers of Darillium," River suggested.
The Doctor choked on his tea. "What's wrong?" she asked him. "Are you alright, sweetie?"
"Fine, fine," responded the Doctor. "Just…a bit hot, that's all." He set down the cup as he recalled that horrible day hundreds of years ago, or hundreds of years in the future depending how you think about it, that he could never forget. He remembered her words and realized that it was now the present, not the past or the future. It was time. "River, what are you doing next week?"
"Oh! I almost forgot. I'm going to visit the largest library in the universe with a group of archaeology majors from the University. Some say that a hundred years ago everyone there mysteriously vanished and were never heard from again. Sounds like your kind of mystery, Doctor. You should come with us!"
"I'll, uh…try to stop by." He wanted to tell her not to go. He wanted to tell her to stay with him in the TARDIS and take her on endless adventures, but he remembered her words: "The last time I saw you- the future you, I mean- you turned up on my doorstep with a new haircut and suit. You took me to Darillium to see the singing towers." He knew that there was nothing he could do. River's death was timelocked. She died to save him because she knew that neither of them would be there to have all the good times that they'd had if she hadn't done so. If the Doctor had sacrificed himself instead that day, he never would have taken Amy and Rory into the time vortex on their wedding night and there would be no Melody Pond, at least not one that was half Time Lord and loved him. All he could do now was return the favor and save her.
"Right, then," he said. "To the singing towers."