When the world's only Yeti in captivity is kidnapped from London Zoo, a group of children succeed in rescuing it.

The Doctor and his companions arrive on a spaceship headed for Earth, populated by natives of Earth from various different eras and commanded by the leaders of the Urbankan race. What are the Urbankans' intentions when they reach Earth?

The Doctor and Adric learn from the wizened Keeper of Traken that a great evil has come to his planet in the form of a Melkur - a calcified statue. The Keeper of Traken is nearing the end of his reign and seeks the Doctor's help in preventing the evil from taking control of the bioelectronic source that is the keystone of the Traken Union's civilisation.

Arnold wants to relax watching TV, but Terry is missing. Terry is the cat of his mother-in-law, who left it with Arnold and his family when she went on holiday. Now Arnold's wife, Hilda, is frantic with worry.