The Doctor, Vicki, and Steven Taylor arrive on an eerily silent planet and encounter curious small robots, which Vicki names Chumblies. It is unclear whether the robots are hostile, when one is disabled by a party of female, cloned Drahvins, from the planet Drahva in Galaxy 4. It is revealed that this unknown planet is also in Galaxy 4, but is not given a name. The Drahvins are dominated by their leader, Maaga, who treats her other warriors with bullying contempt. The Drahvins are at war with the reptilian Rills, the masters of the Chumblies, and both races have crashed spaceships on this planet.
The planet will be destroyed in 14 planetary cycles and, with the Drahvin ship irreparable, Maaga and her warriors are keen to capture the Rill ship, which they believe has been made functional again. Maaga paints a picture of the Drahvins as the attacked species in the scenario, but the Doctor has witnessed some of the Drahvin aggression and is clearly not convinced. He also reworks the probability on the planet’s destruction and calculates it will break up in just two days time. The Doctor tries to keep this new finding from the Drahvins, but Maaga reveals her true colours and forces the truth from him at the point of a gun.
With Steven held as hostage to ensure their co-operation, the Doctor and Vicki are sent by the Drahvins to try to seize control of the Rill ship. The Doctor works out that the Rills are a very advanced species: when he meets one he is impressed, not least by their telepathy. The ugly, horned, ammonia-breathing Rill explains that the Rills have offered to take the Drahvins away with them but Maaga has refused, preferring to maintain the state of war she began when the Drahvins shot down the Rill craft. The Doctor tells the Rills of the true life remaining in the planet and promises to help them escape, since the solar energy converters on the Rill craft have not gathered enough power to effect a lift-off.
The Doctor and Vicki return to the Drahvin ship to find Steven unconscious after Maaga has tried to kill him by leaving him in a depressurised airlock. They all then return to the Rill vessel, where the Doctor successfully develops a power converter linked to the TARDIS, which charges the Rill craft. Maaga leads the Drahvins in a final assault on the Rill craft, but the Chumblies defend their ship long enough for it to power up and leave the planet. One Chumbley left behind to aid the time travellers helps them get back to the TARDIS. Once the ship leaves, the planet explodes, with the Drahvins perishing on the dying world.
The story ends with a lead in to Mission to the Unknown with Vicki looking at a planet, and wondering what is happening on it. The action then switches to the planet, where a man in a jungle is chanting "Must Kill".