The one with an underwater Christmas party.
The fourth in the Adventure Games series, continuing TARDIS's cliffhanger.
London in the 23rd century, now underwater due to Global Warming.
The Doctor and Amy are in the underwater base Poseidon 8, under attack by an alien shark, a strange disease and the Vashta Nerada, who previously appeared in "Silence in the Library" and "Forest of the Dead". The Vashta Nerada also exist on Earth, but the Doctor realises that the ones in Poseidon are far more aggressive than the more timid terrestrial ones that usually live off roadkill. The Doctor and Amy return power to the base, before discovering the source of the strange radiation — the disappeared 20th century ship, the USS Eldridge, whom humans had been creating time travel experiments on. Initially cut off from the TARDIS thanks to the alien shark, the Doctor finds an underground shaft to reach it and board the Eldridge to shut it off.
- Canon Welding: Eldridge hails from The Philadelphia Experiment, in which a radar-cloaking experiment inadvertently sends an unsuspecting destroyer escort forty years into the future. In the film, she ends up, damaged and with casualties, back in 1943; presumably the possibility of time travel intrigued the Navy enough to carry out further experiments.
- Continuity Nod: To "The Stones of Venice", which established that 23rd century Earth has been disastrously flooded. (Though it might be coincidence.)
- Jump Scare: Every time the Vashta Nerada engulf the Doctor or Amy if you fail to steer around them, the screen goes dark and a skull rushes out at you, implying they devoured them. That's because the standard game over effect programmed in of them just falling over in a death sequence doesn't fully convey the horrors of what the Vashta Nerada really do.
- Three Laws-Compliant: The Doctor refrences these when talking to the Base's A.I.