The Doctor and his friends find themselves in the lost city of Atlantis, where the crazed Professor Zaroff has convinced the people of Atlantis that he can raise their sunken city from beneath the sea. However, the Doctor discovers a terrible secret behind Zaroff's plan — a secret that could destroy all life on Earth.

Accident-prone Gil wants a steady job but is dismissed by every company that recruits him due to his unfortunate habit of ending up in sexually embarrassing situations. His girlfriend Annette says that she will marry him only if he can stay employed for at least a week. Hired by Mr Grimwood's cleaning company "Here To Service You", Gil is unwittingly drawn into a series of bawdy misadventures: sharing a bath with a married woman and getting caught by her husband; having a threesome with a hospital patient and a woman doctor, followed by a foursome with Annette, Grimwood and a woman psychologist; awkward encounters with a gay man and a lesbian; and getting caught by Annette playing strip poker with a group of women. Incredibly, Gil remains in his job and Annette marries him, but before the couple can have their wedding night Gil collapses on the bed and passes out from exhaustion.

Receptionist Anne’s new relationship with nascent writer Peter is opening her eyes to a new lifestyle, one far removed from her comfortable middle-class background.

The TARDIS arrives on an extinct volcanic island. Before long, the travellers are captured and taken into the depths of the Earth, where they find a hidden civilisation — the lost city of Atlantis.