The Doctor lands the Tardis in an underground cavern on 26th century Earth. Then the Doctor, Tegan, Nyssa and Adric become embroiled in the search for a team of missing scientists being preformed by the military.

Captain Briggs takes the Doctor prisoner, suspecting him of being behind the murders on board. Meanwhile the Cybermen start the process of taking over the ship with the help of a traitorous crew member. The Cybermen also start reviving other Cybermen who are hidden away as cargo.