When the TARDIS landed on Darillium, the Doctor told River to wait for him outside because he had to go use the toilet. Instead, he sprinted to his bedroom (which he rarely used) and frantically searched for River's data ghost, which would connect to her communicator next week. Well, next week for her- a few hundred years in the past for him. He wished he'd known then who she was. Maybe he could go with her and sacrifice this future version of him, since he had accepted that death was coming in his previous body before his surprise regeneration. He knew that he couldn't do so, though, because having two versions of himself in such an unstable place at once could create a massive paradox or completely change the course of events with devastating consequences. He had done it before while saving Gallifrey…but no, he shouldn't meddle with this. He had some great times with River, but it was time for him to move on.
The Doctor found the data ghost behind a photograph of him and River at Asgard. He'd taken her on a picnic that day- one of the dates she had mentioned at the library. He then carefully inserted the device into his sonic screwdriver and ran back to the console room and out the door. River looked so beautiful leaning against his blue box looking up at the stars. He could see the towers in the distance and faintly heard their singing.
"There," he said, straightening his tie. It was difficult getting used to wearing a regular tie again after so many years of bowties, but he needed to switch things up a bit. "Let's go see those towers, shall we?"
They proceeded hand-in-hand through a series of brightly lit streets crowded with creatures from a thousand different galaxies who had come to hear the towers sing. Some were purchasing food or souvenirs from street vendors, while others were mingling and laughing with one another. As the Doctor got closer to the towers with every step, their songs became more clear to him. He wondered how the residents were able to sleep through the constant noise.
River wanted to get as close as possible, so they walked all the way up to the rope with the "Authorized Personnel Only" sign that encircled the towers. Normally he would ignore signs such as those, but he didn't feel like arguing with the officials and making River run for their freedom tonight. He sat down on the soft grass and she followed his lead.
"Are you feeling alright, Doctor?" she asked. "You seem rather quiet tonight."
"I'm just…enjoying the music," he replied with a forced smile. He couldn't believe that after so many companions lost, he was losing the most important one of all. He couldn't even visit her in the past because she'd be with his previous self.
River rested her head on his shoulder and he ran his new fingers through her mess of curls. The towers were singing a traditional romantic ballad, accompanied by fireworks. A few tears came to his eyes. One landed on River's cheek and she glanced at him. "Really though, what's wrong?"
"Nothing." He wiped the tears away.
"The towers sang and you cried," she had said. "You wouldn't tell me why, but I suppose you knew it was time. My time. Time to come to the library."