Parts: 6
Writer: David Whitaker (from a story by Kit Pedler)
Airdates: 27 April 1968 - 1 June 1968
Production Code: SS

Doctor: second
Companions: Jamie, Zoe

Creature Feature: Cybermen

Story: Having said their fond farewells to Victoria, the Doctor and Jamie try to muster some excitement for their latest destination - but things suddenly turn desperate when the fluid links burst, and the Doctor and Jamie are forced to evacuate the TARDIS before the mercury vapor suffocates them. They find themselves onboard an abandoned rocket drifting in space. As the rocket approaches a wheel-shaped space station, the Doctor and Jamie discover that it is not quite abandoned - a servo-robot is onboard, and before Jamie can destroy it, it has attacked the Doctor.

The crew of the station are concerned about the rocket's presence, worried that it will drift toward them and collide. Fortunately, Jamie is able to alert the crew to their presence on the rocket before it is destroyed, and some of the crew members take the Doctor to the sickbay on the Wheel. But the damaged TARDIS is still aboard the rocket, and the crew still intend to destroy it - so Jamie takes drastic action and disables their defensive laser. This renders the Wheel vulnerable not only to the rocket, but also to the meteorite storm that has been deflected their way by a recent supernova.

But while the Wheel staff are preoccupied with the damage Jamie has done and the threat posed by the rocket, something even more sinister is happening without their knowledge. The Wheel is gradually infiltrated by Cybermen, who immediately set to work sabotaging the space station with the help of their Cybermats. With the meteorite storm approaching, the Wheel's commander becoming unstable and the Cybermen slowly taking over the crew, the Doctor and Jamie must regain the crew's trust - with the help of their parapsychologist, Zoe - and figure out how to stop the Cybermen's deadly scheme before they are all destroyed.