A mystery virus is wreaking havoc among the crew of the Earth's weather control station on the moon. While investigations into the strange disease are in progress International Space Headquarters Earth puts the entire Moon base into strict quarantine - the Doctor and his companions included! To make matters worse, Moon base personnel inexplicably vanish and vital weather control equipment is sabotaged. Who is responsible? The Director of the base suspects the time-travellers. The Doctor fears that the ruthlessly evil Cybermen are at work...