The Doctor appears to have transformed into an entirely different person, but Ben and Polly are unsure whether this strange man really is the Doctor or not. They soon have other matters to worry about, as the TARDIS takes them to a human colony on the planet Vulcan. Rebel activity is causing trouble for the colony governor, an Earth Examiner has recently been murdered, and the colony scientist Lesterson is obsessed with his experiments on an alien capsule unearthed from the mercury swamps. The Doctor takes the Examiner's place in order to investigate his murder, but soon discovers that the alien capsule is far more significant – for it contains dormant Daleks which Lesterson has woken. Lesterson believes that the Daleks are harmless robots which can be used to serve the colony, while the rebels are far more interested in the uses of the Daleks' in-built weaponry. Nobody will listen to the Doctor when he warns them that the Daleks have their own agenda – one which could result in the extermination of all human beings in the colony.