The Doctor and the newly formed UNIT must stop a Cybermen Invasion and a Sinister Industrialist in league with them

The Doctor and Jamie are taken prisoner by their mysterious observers and have a reunion with an old friend while Zoe and Isobel carry out their own investigation of IE.

The Doctor and Jamie return to the IE factory in search of Zoe and Isobel, where they meet with Professor Watkins and fall foul of Vaughn.

The Doctor and Jamie enlist the help of UNIT to rescue Zoe and Isobel before trying to find out what Vaughn is really up to.

The Doctor and Jo head to Stangmoor Prison to investigate the Keller Machine, which drains the evil impulses from prisoners' minds, while UNIT are handling security at the first world peace conference.

The Brigadier has recaptured Stangmoor but the Master still has control of the Thunderbolt and the Doctor must find a way to stop him and destroy the Keller Machine.