The Doctor dreamed.
He didn't often dream, as Time Lords didn't often dream, but seeing as his mind was slowly reconstructing events of his life, he supposed he shouldn't have been too surprised.
The first dream was pretty standard brain-dumping. The Doctor was late for class and looking for Zac Efron at a Beatles concert, and Ringo Starr chased him around with a hypodermic needle. Dreams of that level of absurdity were pretty easy to ignore.
The second dream, not so much. He was standing on a beach. It was Norway, or was it Boeshane? It was hard to tell. Everything was fuzzy at first, but the further he walked along the beach, the clearer it became. The single sun (or the twin suns) obscured by clouds, the biting cold. He tucked his hands into his pockets for warmth, but it kept getting colder. The lights kept fading.
He spun around, and there was Jack. Standing with his feet in the icy water. The Doctor knew Jack was supposed to be a fact, but he couldn't shake the strong, petrifying worry that Jack might die of hypothermia. The tide rose quickly, too quickly.
"Jack," the Doctor said. "Get away from there."
"Doctor, it's coming," Jack said. Only, Jack didn't call him Doctor. He called him his name, the name he never used, the name he'd all but forgotten after so many years.
Curiosity overtook worry for one moment. "Who's coming?"
Jack shook his head, but didn't move. The water level kept rising, quickly overtaking his companion. The Doctor raced towards the shoreline, but it wasn't getting closer.
"Jack!" he called. "Jack!"
"He will knock four times," Jack said, firmly, right as the water overtook his head.
The Doctor sat up from his curled position in the center of the Zero Room. It was colder than he remembered, but he could feel the TARDIS had fallen back into a restorative hibernation. Jack hadn't come back and from the amount of stubble on his chin, the Doctor imagined he'd actually slept a good few hours this time.
He struggled to his feet and hobbled to the door. With a tug, it came open to reveal the dark, cold corridors lit by fuzzy, organic nightlights. He headed back towards the bedrooms, intent on his own room for a shower and a long-needed shave. He stopped as he passed Jack's room. Would he be in here? Would he be gone?
He gave the door a quiet knock.