Sarah discovers that the spaceship passengers are all victims of a deception.
The Doctor soon twigs that an over-zealous Yates is the UNIT mole. Sergeant Benton lets the Doctor escape, for which Finch threatens a court martial. The Doctor uses his freedom to track down more monsters, but when he is recaptured, the Brigadier asserts his authority and takes the Doctor into UNIT custody rather than the regular army’s.
Meanwhile, Sarah has escaped from the fake spaceship. She has learnt its true nature. She is caught by Finch, who returns her to Whitaker’s custody. While she is away, Mark works out that the ship is a fake and tells the other passengers, but he is not believed. When Sarah is returned to the ship, Mark and she use the fake airlock to convince Ruth and the others of the deception.
The Doctor encounters another time eddy and is faced by more dinosaurs.