A spaceship lands on an apparently deserted island on the planet Dulkis. The ship's two occupants, Probationer Toba (Kenneth Ives) and Navigator Rago (Ronald Allen) are members of a race called Dominators. Their ship has absorbed the high levels of radiation on the island. They despatch their robot Quarks (John Hicks, Gary Smith, Freddie Wilson. Voices: Sheila Grant) to prepare five drilling sites. 

Three Dulcians - Wahed (Philip Voss), Etnin (Malcolm Terris) and Tolata (Nicolette Pendrell) - are on an excursion trip organised by Cully (Arthur Cox). Cully tells them they are approaching the Island of Death - an atomic test site - but his passengers doubt this as the radiation counters are not registering. Their ship crashes and all but Cully are killed by the Quarks. 

The TARDIS arrives on the island. The Doctor tells his friends that Dulkis is a quiet and peaceful planet where they can have a holiday. Cully watches from hiding as the Dominators blow up his ship. The Doctor, Jamie and Zoe hear the explosion and go to investigate. Along the way they find a ruined war museum which is full of old weapons and are taken prisoner by three figures in radiation suits. 

They are taken to an observation centre where, after discovering that they are not contaminated, their three captors reveal themselves to be two students - Teel (Giles Block) and Kando (Felicity Gibson) and their teacher Educator Balan (Johnson Bayly). They are on the island studying the effects of the radiation from an atomic explosion set off as a warning about such weapons many decades before. 

Cully passes the TARDIS on his way to the museum and overhears Toba and Rago discussing whether to destroy it. The headstrong Toba wants to blow it up but Rago orders him to conserve their dwindling energy supplies. 

Cully meets Teel and is taken to the observation centre where he warns Balan and the Doctor about the aliens and their talk of destroying the TARDIS. The Doctor and Jamie rush to check while Balan remains unconvinced by Cully's tale. He reveals that the man is the son of Dulkis's leader Director Senex (Walter Fitzgerald). Balan tries to contact the Dulcian capital but has great difficulty due to strange interference. When they make contact the Dulcian leaders order that Cully and Zoe should travel to the capital by transit capsule. 

Jamie and the Doctor find the Dominator's ship and are captured. Rago orders them to be tested for strength and intelligence to see whether the natives are suitable as slaves. The Doctor pretends to be stupid by deliberately failing the tests and they are declared useless and freed. The Doctor however learns that the Dominators are attempting to mine some kind of element from the planet. 

Cully and Zoe arrive in the capital within minutes and are taken before the council. It is the Dulcian way to debate every point and fact no matter how trivial and Senex disbelieves Cully's story of aliens. Cully tells Zoe he plans to get proof by stealing a transit capsule and returning to the island. 

Whilst they return to the island, the Doctor and Jamie travel to the capital. The Doctor warns Senex and the council about the Dominators but the Dulcian leaders cannot decide a course of action preferring to do nothing. Whilst the Doctor and Jamie are away the Dominators and their Quarks invade the observation centre and take Balan, Teel, Kando, Zoe and Cully prisoner. 

Rago orders the prisoners to be put to work clearing the drilling sites to conserve the Quarks' energy. In the capital, the Doctor and the council try to contact the observation centre and see a Quark on guard in the building. The Doctor and Jamie race back by transit capsule. Jamie points out that they are heading straight towards the waiting Quark and the Doctor tries to fly the capsule manually resulting in a heavy but safe landing away from the observation centre. 

Zoe and Cully are working on a plan of escape. Zoe creates a diversion while Cully runs into the museum and grabs a laser gun. He finds that he lacks the courage to shoot. Jamie finds Cully in the museum. The young Scot has no problems using the gun to destroy a Quark. Enraged, Toba orders the other Quarks to fire on the museum building until it collapses. 

The Doctor is captured and taken back to the Dominators' ship along with the Dulcians. Toba kills Balan and threatens to kill the Doctor but is stopped by Raga who is furious at Toba's disobedience. Teel and Kando are ordered back to the drilling site whilst the Doctor and Zoe remain on the ship. The Doctor discovers that the ship is capable of storing vast quantities of radiation to power both the ship and the Quarks. 

Jamie and Cully find an atomic shelter below the museum where they hide as the building is demolished. They manage to escape and rescue the others from the Quarks, returning everyone to the shelter to hide. 

Rago goes to the Dulcian capital and orders the council to obey him. Chairman Tensa (Brian Cant) is outraged and is killed by a Quark. Rago tells Senex to obey the Dominators and to prepare some of the population for transport off the planet as slaves. 

The Doctor has deduced that the Dominators intend to turn Dulkis into a ball of radioactive matter which will be used to fuel the Dominators' fleet. They intend to use rockets to crack the planet's crust and then to drop an atomic seed device into the planet, thus converting it to fuel. 

The Doctor organises the other to begin to dig a side tunnel from the shelter to the central bore hole so that the Doctor can grab the seed device and prevent the planet's destruction. Meanwhile Cully and Jamie use guerrilla tactics to destroy the Quarks thus slowing down the drilling process. 

Low on energy, Rago orders the remaining Quarks to return the ship in preparation for the planet's destruction. The Doctor manages to obtain the atomic seed device and places it aboard the Dominator ship just before it takes off. The ship explodes in the air when the seed device is activated. 

The Doctor tells Cully and the others to get back to the capital before the other Dominator rockets explode and create a volcano. Back in the TARDIS the Doctor is forced to move quickly as the lava approaches the ship.

_{Synopsis from Doctor Who: The Second Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}