When a Nestene Energy Unit is stolen and a radio telescope sabotaged, the Doctor is contacted by a Time Lord who tells him his old rival the Master is responsible.

Attempting to find out where Jo encountered the Master, the Doctor heads to the circus where he falls foul of Rossini.

The Doctor and the Brigadier head to Farrel's factory but the Master and the Autons have already left and are distributing plastic daffodils around the country.

The Doctor discovers the plastic daffodils are designed to fire a spray that asphyxiates people, causing havoc prior to the Nestene's arrival, but before he can alert anyone he and Jo are captured by the Master.

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 Doctor is recalled by UNIT to help investigate Cheng-Tiek's murder but is unaware that Chin-Lee is under the control of the Master.

Realising Chin-Lee has been using the power of the Keller Machine to carry out assassinations, the Doctor heads back to Stangmoor, only to find the Master has teamed up with Mailer to stage another riot.

With the Doctor and Jo being held prisoner, the Master convinces Mailer to help him steal the Thunderbolt.

The Doctor is forced to help the Master control the mind parasite while Yates finds out where the Thunderbolt has been taken only to be captured by the Master's men.

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.

When a mysterious object lands near the Nuton power complex, UNIT investigate and encounter the Axons, a group of aliens who offer the mineral axonite, which can duplicate matter, as a gift.

The Doctor and Winser begin to analyse axonite while Chinn plots to secure its use exclusively for Britain.

The Doctor and Jo are being held prisoner by Axos while Chinn is forced to organise the worldwide distribution of axonite and the Master returns to the power complex to try and steal the TARDIS.

The Doctor and Jo escape from Axos but then the Doctor offers the Master a deal; if he helps him repair the TARDIS, they can flee Earth together.

The Time Lords take control of the TARDIS and the Doctor and Jo arrive on an Earth Colony in the 25th century, where a ruthless mining company are using a dinosaur-like creature to force the colonists to leave the planet.

The Doctor discovers an Interplanetary Mining Corporation ship has landed on the planet in order to mine its duralinium, while Norton begins causing trouble at the colony.

The Doctor makes it back to the colony and warns Ashe what Dent is up to but when Jo and Winton try to find proof they are captured.

The Doctor descends into the Primitive city to try and find Jo while the colonists receive a visit from the Adjudicator.

The Master offers to help the colonists in exchange for information on the Primitive city and forces the Doctor to act as a guide by taking Jo hostage.

Earth, the near future. When an archaological dig at the village of Devil's End goes disastrously wrong, an ancient power begins to revive. Can even the Doctor withstand the power of the Daemons? And just what is the local vicar up to in his spare time?

With the Doctor frozen by the energy release from the barrow, Jo calls Yates and Benton for help, but when the Brigadier tries to join them he finds the village surrounded by a heat barrier.

The Doctor explains to his friends that the Master is trying to gain the power of the last of the Daemons, a race who influenced Earth's development, while the Master brings the rest of the village under his control.

The Master orders Bert to set a trap for the Doctor at the village while a concussed Jo makes for the cavern.

The Master decides to sacrifice Jo to Azal and when the Doctor and his friends try to intervene they come under attack from Bok.