Another adventure for everyone's favourite Time Lord. The Doctor (Jon Pertwee) and Jo (Katy Manning) travel to Stangmoor prison to investigate a machine which can remove evil from the minds of criminals. However, the man behind the scheme - Professor Kellerman - is in fact the Doctor's arch-nemesis, the Master (Roger Delgado), and is planning to disrupt a United Nations peace conference. Inside the Master's machine is an alien parasite which can make people see their greatest fears, and is growing stronger all the time. Can the Doctor stop both the Master and the unearthly parasite?

The last adventure to star Jon Pertwee. To save the Universe, the Doctor must travel to the planet Metebelis Three, where he discovers an invasion plan of Earth by the Metebelis Spiders, under the leadership of the Great One. To save the planet the Doctor must absorb a lethal dose of radiation, triggering his third regeneration...