The TARDIS arrives on the planet Dulkis where the Doctor, Zoe and Jamie decide to have a quiet holiday. Only to find the Evil Dominators and their robotic slaves, The Quarks have arrived on a radioactive island, where they plan to detonate a bomb in the planet's core, which will turn the entire planet as source of radioactive fuel for their invasion fleet.

The Doctor and Jamie are captured by the Dominators, who subject them to an intelligence test, while Zoe and Cully attempt to convince the Dulcian council of the threat.

Zoe and Cully are captured by the Dominators and put to work alongside the survey team while the Doctor and Jamie decide to return to the island to help.

The Doctor and Zoe are interrogated by Rago, who is planning to present his terms to the Dulcian council, while Jamie and Cully try to find a way to strike back against the Quarks.

Jamie and Cully manage to rescue the Doctor's party and together they try to find a way to stop the Dominators before they destroy the entire planet.

While the Doctor plans a holiday to Metebelis Three, Jo and the Brigadier are intrigued by a mysterious death at a coal mine in Llanfairfach, which is being blamed on local company Global Chemicals.

The Doctor and the Brigadier attempt to rescue Jo and Bert from the mine but Global Chemicals prove distinctly uncooperative.

The Doctor and Jo manage to escape from the mine with a maggot egg but Stevens is determined to stop them analysing it.

The Brigadier calls in a full UNIT force to deal with the giant maggots while the Doctor infiltrates Global Chemicals to find out what is really going on.

The Doctor manages to escape from BOSS with help from Yates and discovers Cliff has been infected by a giant maggot.

The Doctor attempts to destroy the giant maggots and cure Cliff but the real threat is BOSS, who is poised to take over all the world's computers.