An alien craft bearing the imperious and ruthless Dominators arrives on the peaceful planet of Dulkis. The senior, Navigator Rago, is at odds with Probationer Toba, making for an uneasy partnership. The craft lands on the Island of Death, a nuclear test site that now houses an anti-war museum, and soon absorbs all the radiation on the island. The robotic Quarks are then sent out by the Dominators to prepare bore holes into the planet’s crust, since the Dominators wish to convert the planet into rocket fuel. Toba uses the Quarks to fire on and kill three indolent, rich adventure seekers who stumble across his project. Their pilot Cully, however, survives by hiding himself away, though the craft that brought him to the island is destroyed. Rago is furious that these potential slaves have just been wasted.
The TARDIS has arrived on another part of the island and the Second Doctor and his companions Jamie and Zoe begin to look around. The Doctor has been to Dulkis before and is looking forward to a good holiday when they hear the explosion of the craft being destroyed. They take shelter in the museum building where they meet three other newly arrived Dulcians, Educator Balan and his young charges Teel and Kando. All are puzzled that the radiation reading on the Island reads nil, since it should be radioactive after the nuclear explosion 172 years earlier, and since that time the Dulcian civilisation has rejected all weapons. Cully arrives too, and tells them about the murderous Dominators and their robots. Balan does not accept this: he knows the son of the Director of the ruling council is well known as a con artist.
The Quarks have meanwhile marked out their drill sites, and begin work on drilling the outer bore holes. However, their power supply seems limited, and Rago is very keen to conserve their energy supplies to vital activities only. This actually saves the Doctor and Jamie when they are captured by a patrol of Quarks rather than killed, and taken to the Dominator ship for questioning and scanning. A scan of Jamie is presumed to apply to them both and to the Dulcian race as a whole, who are thus described as possible but not definite for conversion into a slave force. When it is time for an intelligence test the Doctor feigns stupidity to prove their worthlessness. He also fails to use a weapon from the Dulcian museum, falsely claiming such military technology has been lost to the Dulcians. The Doctor and Jamie are freed as worthless idiots.
Cully has contacted his father, Director Senex, who orders him to recharge Balan’s travel capsule and use it to return to the Capital City. He takes Zoe with him on the journey to the Council Chamber, where the discussion lacks focus and purpose. Senex refuses to believe that Cully is telling the truth, despite Zoe’s protests, so Cully steals a travel pod and heads back to the island with Zoe to get proof of their story. At the same time the Doctor and Jamie take Balan’s pod to the Capitol City. They are angry that Cully and Zoe have been allowed to return to danger and have real trouble convincing the Council of the threat which the Dominators pose, with the Dulcians repeatedly demonstrating a totally pacifist stance. The true danger is only revealed when the Council obtains a visual image of the survey station near the museum which the Quarks destroyed on Toba’s instructions – another case of wasted power in the eyes of an increasingly exasperated Rago.
The Dominators have meanwhile captured Balan, Teel and Kando, using them for further tests on their species, which the Dominators assume are a higher life-form than the Doctor. They are assigned to work as slaves in the drilling sites under the supervision of the Quarks. Zoe and Cully are soon captured too and added to the slave force, but find Balan and Kando totally opposed to using force against the overseeing Quarks. Over time, however, they all start to falter under the weight of the burden of digging the bore hole, with Balan collapsing first. Cully manages to sneak away back to the museum and capture a laser weapon stored there as an exhibit.
The Doctor and Jamie have meanwhile taken control of a travel pod and steer it back to the Island of Death. Jamie links up with Cully at the museum while the Doctor is captured by Quarks and taken with the slave force back to the Dominator ship. Cully uses the gun to destroy a Quark, prompting another of Toba’s rages. The museum is destroyed in retaliation, which once more enfuriates Rago. He orders the Quarks to hold the Doctor and Zoe for further tests while Balan, Kando, and Teel are sent to excavate to the central bore site.
Jamie and Cully survived the explosion in the museum and are hiding in a nuclear bunker below the main building. After a struggle they succeed in opening the hatch above them. They survey the area and succeed in crushing a Quark with a boulder. An angry Toba is alerted via an alarm on the ship and heads off to investigate how another Quark has been destroyed, leaving the Doctor and Zoe free to roam the Dominator ship and work out more about its power source. Jamie and Cully remain free and use the opportunity to attack other Quarks in a series of guerrilla raids.
The Dulcian Council debates the situation and not even Tensa, Chairman of the Emergencies Committee, can spur them into decisive action. The key moment comes when Rago himself uses the travel pod appropriated by the Doctor to travel to the Capitol with a Quark. The robot kills Tensa on command. Rago says that the fittest Dulcians will now all be converted into a slave force to use on the Dominator homeworld, with the Quarks used there redeployed to the front line of their expanding empire. Those not selected will be left on Dulkis to die, as the planet is doomed. Having finished his ultimatum, Rago stalks away.
Toba now returns to the ship, with his slaves in tow, and demands to know who destroyed the Quark. Balan is killed for refusing to answer, and the Doctor is selected to die next. Luckily for him Rago returns and is incensed that Toba has wasted more lives and not even begun work on digging the central bore hole even if the four perimeter ones have been completed. Toba is sent to complete the drilling and prepare the bore rockets, using the Quarks and the Doctor, Zoe, Teel and Kando as slaves; while Rago focuses on a precious seed device that is intended to be dropped down the central bore hole. Rago also hears from the Fleet Leader that no Dulcian slave force is to be assembled: all the Dulcians are now to stay on the planet to die when it is destroyed.
The dig proceeds with the Doctor and the other slaves making some progress, but when Toba abandons his watch post Jamie and Cully seize their opportunity and disable another Quark to enable the freedom of their friends. The Doctor has now worked out the Dominator scheme: a nuclear fission seed will be dropped down the bore hole which will convert the entire planet into a radioactive mass ideal for use in powering the Dominator fleet. The four outer bores are for launching rockets through the planet’s crust – which is especially thin at the point of the Island – and will cause volcanoes to erupt; while the seed will then be used to detonate the volcanoes being created and begin a radioactive chain-reaction. Their collective response is to capture the seed and they begin digging a perpendicular tunnel which should reach the central bore hole and enable them to steal the deadly device before it can detonate. However, there is little that can be done to stop the volcano creation. Jamie and Cully support this effort by continuing to attack and destroy Quarks with homemade bombs. This has the desired effect of totally fracturing the relationship between Rago and Toba, as they argue about priorities rather than prioritising the dig, but yet the bore hole is still ultimately completed and the seed primed.
The access tunnel scheme is successful and the Doctor intercepts the seed during its descent, telling his friends that it cannot be defused. Cully, Teel and Kando are told to flee in the remaining travel pod, while Jamie and Zoe are sent to the TARDIS to wait. The Doctor runs to the Dominator ship and manages to smuggle the seed on board before the craft lifts off. It soon departs and the Dominators’ last vision is of the seed device rolling on the floor toward them. The Doctor watches the Dominator ship being destroyed and then heads back to the TARDIS where he and his two companions must depart in a hurry to avoid the advancing lava flows from the new volcanoes...