This text explores a conflict on the planet Solos, where human colonial 'Overlords' under the control of the brutal Marshal clash with the native Solonians. The Marshal seeks to make the planet habitable for humans, despite the harmful effects on the Solonians, who are undergoing a natural but accelerated metamorphosis into 'Mutants' due to the humans' atmospheric experiments and radiation from depleted mines. A conference for Solonian independence is disrupted by an assassination orchestrated by the Marshal. The story follows the Doctor and his companion Jo Grant, who become entangled in the escalating tensions, attempting to understand the Solonian mutations and expose the Marshal's actions, ultimately aiding in the revelation that the "Mutants" are not diseased but are evolving to survive the changing environment.