The Tardis lands on the deep space haulage ship Dreadnought which has been taken over by Cybermen. The Doctor befriends a survivor called Stacy Townsend. The Cybermen capture the Doctor and Stacy and attempt to turn the Doctor into a Cyberman in order to steal the secrets of time travel. However a distraction caused by a man called Bill who has partly converted into a Cyberman allows the Doctor and Stacy to escape. The Doctor reverses the polarity of the TARDIS's neutron flow which destroys the Cybermen.

The Doctor and Stacy arrive on Mars and are captured by Ice Warriors. The Doctor becomes involved in a power struggle between Lord Uzoxx and Lord Artix. The Doctor, together with Stacy and an Ice Warrior called Ssard are attcked and become separated. The Doctor rescues the Martian Prince Izaxyrl from the clutches of Artix.

Following the events of Descendance, Stacy and Ssard are being held captive by Artix. The Doctor and Izaxyrl rescue Stacy and Ssard. Uzoxx's wife Luass is in league with Artix and attempts to frame the Doctor with murder. However Artix confesses their plans and Luass is killed by Uzoxx. Ssard joins the crew of the TARDIS.

The travellers arrive in Victorian London and discover a space ship which is invisibel to the people of the time. They meet the ships inhabitant P'fer'd who is an Equanoid, a horse-like alien. P'fer'd tells the Doctor that his wife M'rek'd is missing. Stacy, along with some street urchins rescue M'rek'd from a freak show. However the owner of the freak show is a shape shifting alien and Stacy and the urchins are attacked. M'rek'd and P'fer'd are reuinited and Stacy returns.

The Doctor discovers that Stacy has been replaced by a shape shifter. The Doctor traps the four shape shifters in a sonic bubble and returns them to their home planet. The Doctor, Stacy and Ssard continue on their travels.