|Edition: || UK (VHS)|
|Released: ||February 2003|
|Publisher: ||BBC Video|
|Format: ||1 x VHS PAL|
The Time Lords choose the Doctor for a special task — a mission so secret that neither the Doctor, or his assistant Jo Grant, know the true nature of their assignment...
Far in the future, the planet Solos is claiming independence from Earth's vast galactic empire. A huge space station — Skybase — orbits the planet, forming a bridgehead for the empire's control of Solos, and is commanded by the Marshal, who is determined to retain his power over the planet at any cost.
Solos's atmosphere is poisonous to humans in the daylight, and the Marshal is behind secret experiments to change the planet's atmosphere into one more similar to that of Earth. But down on the planet, changes are happening amongst the native population. A disease that causes mutation has appeared and is spreading fast.
The Doctor finds himself in a race against time to save Jo's life, to find a solution to the mutants, to complete the Time Lord's mission, and to thwart the Marshal's plans...
This story was broadcast on BBC1 between 8 April - 13 May 1972.