The Doctor (Matt Smith) and Amy Pond (Karen Gillan) arrive on the Starship UK, a colony ship taking the UK population away from deadly solar flares on Earth. A three-faced Winder (Chris Porter) takes Amy to a voting booth when she finds a tentacled creature in a hole. She is asked to choose between remembering or forgetting what she saw. She chooses to forget and has her recent memory wiped. The Doctor chooses protest and the two of them are sent into the maw of the creature. The Doctor makes it vomit and they are able to escape. They meet Queen Elizabeth X, or Liz (Sophie Okonedo), the ruler of the ship.
The Winders capture them again and they are taken to the Tower of London. The ship is riding a great Star Whale, controlled by painful electrical impulses to the brain. Liz finds a message from her younger self allowing this. She has been living and forgetting the same 10-year reign for 200 years, always finding out the truth about the poor tortured whale and choosing to forget it. She now has to choose to forget again or abdicate, which will cause the ship’s destruction.
The Doctor reluctantly decides to make the whale brain-dead, to end its suffering and yet save the population. Amy recalls something she heard and forces Liz to abdicate. The whale continues and even speeds up. It seems the creature willingly helped the starship UK for the sake of the children and never had to be tortured in the first place. After the Doctor and Amy return to the TARDIS, they receive a call from Winston Churchill in the Cabinet War Rooms during World War II and see the shadow of a Dalek.
The purpose of the episode is to show how important it is for the Doctor to have a humane companion. It received generally good reviews, though Stephen Moffat himself thought it a bit of a mess. It reminds us of the Cities in Flight novels of James Blish.