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Arriving on an arid planet, the Doctor and his companions encounter the Drahvins, a species stranded on the planet, which is soon doomed to explode. But the only functioning ship is in the hands of another mysterious species, the Rills.

With Maaga holding Steven as hostage, the Doctor and Victoria have no choice but to investigate the Rill's spaceship. There, they discover the operation behind the Chumbelys, and come face-to-face with the mysterious Rills.

While trapped in the Rill spaceship, Vicki discovers that the Rills aren't what the Drahvins said they were. Meanwhile, Steven is trapped in the air lock after failing to escape Maaga's grip, and the oxygen is running out.

With the planet about to explode, the Doctor and his companions are racing against time to save themselves and the Rills. But Maaga and the Drahvins have plans to steal the only working ship on the planet and being the only ones on it.