The inhabitants of Mars vow revenge when human astronauts land on the planet and attack their city, mistaking the Martian city's surveillance towers for weapons.

Captain Scarlet's power is put to the test when he joins Spectrum's operation to protect the Mysterons' next target for assassination, the Director General of the United Asian Republic.

After the Mysterons threaten to destroy London and a lorry carrying an atomic device goes missing in the city, the only clue to its whereabouts is the driver, found unconscious in the street.

After a failed break-in at a nuclear power plant, a manhunt begins for former Spectrum officer Captain Black.

The Frost Line Outer Space Defence System in Canada is threatened by the Mysterons, so Captain Scarlet and Lieutenant Green are sent to one of the bases, Red Deer, to investigate.

Wanting to protect the other Spectrum agents, Colonel White decides to leave Cloudbase and go into hiding after the Mysterons fail in their initial attempt to assassinate him.