The one where Einstein is Cthulhu.
The BBC hosted a competition which allowed children across the UK to try their hand at writing for Doctor Who, with the winners to see their creation come to life. Said winners were four students at Oakley Bay Church of England Junior School. The episode witnessed the Doctor in his ongoing quest to just have a fez, Albert Einstein and an Ood.
The Doctor enters the TARDIS control room giddy to finally get his fez back. He places it on a lever, then accidentally trips and pushes the lever, making the fez disappear. After the fez disappears, however, the Doctor's old friend Albert Einstein appears inside the TARDIS wearing an identical fez and holding a beaker of green liquid. Einstein talks about trying to figure out time travel to the Doctor and says that the green liquid may be the key ("No," the Doctor says). As Einstein walks away, the green liquid bubbles onto his face and transforms him into an Ood. Oodstein walks toward the Doctor and repeats the phrase "Death is the only answer," before the Doctor manages to transform him back into Einstein. The Doctor steals his fez and sends Einstein back to his own time.
- Arc Words: It had been written by children, but the Ood proclaiming that "Death is the only answer" to the Doctor, who had yet to face his own death is a weird coincidence.
- Beethoven Was an Alien Spy: Well, Einstein spied on an alien, trying to understand time travel. Not to mention, the Doctor nicked his toothbrush. And his fez.
- Canon Discontinuity: It's a very weird special written by schoolchildren in a contest. The lack of future references to it shouldn't be surprising.
- Cliffhanger: The episode ends with the green liquid crawling on the floor and the Doctor doesn't seem to notice it.
- Continuity Nod: "I'll have you know this 'old trash' will be around until the end of time. In fact, it was."
- Crack Fic: Yeaaaaaaaaah...
- Development Gag: Probably not, but still an adorable coincidence. When Matt first got the part as the Doctor, he gave an interview that said in order to prepare for the role, he wrote fan-fiction where the Doctor met Albert Einstein. It's possible that the students saw that interview and decided to run with it, but even if they didn't, you have to wonder how Matt felt about the whole thing.
- Einstein Hair: "You should keep it. Looks more science-y."
- Noodle Incident: Einstein has apparently stolen the TARDIS before.
- Red Eyes, Take Warning: As always, the Ood.
- Running Gag: First line of the episode: "Oh, I've missed you! I can't believe River blew you up!" — the Doctor, to his fez.
- Title Drop: "Death is the only answer."