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.
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.
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.
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 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 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 is using a dinosaur-like creature to force the colonists to leave the planet. The Doctor and Jo learn that The Master is on the planet and is in search of a doomsday weapon that is hidden somewhere on 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 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.
The Doctor and the Master are taken into the Primitive city, where the Master hopes to find the doomsday weapon, while Dent forces the colonists to leave the planet even though their ship is likely to explode.
The Doctor and Jo race to the village of Devil's End to prevent what the Doctor believes could be a disaster. With a midnight excavation due to take place, mysterious events are happening - not helped by the Reverend Magister.
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.
