The Doctor paced around the console of the TARDIS. Amy and Rory were gone. Everything was lonely. It took him a moment to remember, but he realized why he likes to bring friends with him in his travels: with people around to talk to, he rarely is faced with his mina alone.
Although it was weeks ago, months even, the Doctor found his mind drifting to thoughts of a certain girl… A girl who was not quite alive, but not quite dead. A girl who was not quite human. A girl who went by the name of Tyler Donna Jones.
He continued to pace, letting his hands drift aimlessly over the TARDIS console, wondering what had become of Tyler Jones. Was she still alive and well? Or had she been injured in some way? Without injury, she'd go on forever, but she would still have to be careful. The Doctor regretted having no way to contact her. The feeling of regret swarmed over him, threatening to stir back memories even further back, yet not so far back at all.
Berlin, 1938… Melody Pond had just regenerated... He was dying… And asked the TARDIS to create voice interface. But not of him… Of someone he liked… Rose Tyler.
Seeing her face, after he knew he'd never see her again… It cut him in two to be reminded of that fact. The regret turned into guilt, for he had, at one point, put Rose, herself, through that turmoil.
It was lifetimes ago, when the Doctor had sent Rose away, putting her on the TARDIS to return her home, knowing she'd die if she stayed to face the Daleks on the Game Station.
And the Doctor had sent her a message. A hologram had appeared to Rose, showing the Doctor, bidding his final goodbyes. Rose had been afraid, skirting the edge of the room tog et away from the locked-gaze of the hologram's eyes. But the Doctor, knowing Rose so well, set the recording to look to the side at the end, predicting Rose's actions. It was designed to look real… As if he was actually there… But it was all pre-recorded… Predicting Rose's reactions….
And then it clicked. Rose's last message to the Doctor. She hadn't been communicating with him at all, he realized. It was just a package of psychological content and faux-consciousness data. A message, designed to predict with perfect accuracy, how the Doctor would react. And it had worked.
The message was "He will knock four times."
The Doctor had suspected that it meant something sinister. He assumed it meant that he would soon have an encounter with the Master… Some time in the future. But it wasn't. It was a pre-recorded message, and the message arrived too late. It was supposed to be delivered sooner, before his regeneration, to warn him of the Master.
But something had gone wrong with the data patterns. That's the problem with cross-universal communication: you never can get it quite right.
The Doctor sighed, and closed his eyes, trying to block out the guilt, the regret, the sorrow. He fiddled with the TARDIS console, wondering where to go next, needing a new destination.
Because, knowing his last goodbye with Rose Tyler turned out to be nothing but a re-recorded message… He needed a distraction from how his heart was breaking all over again.