After landing on what appears to be a dead planet, the Doctor decides he wishes to explore. Barbara and Ian are not to pleased by this and in order to get what he wishes, the Doctor sabotages the TARDIS by taking the fluid link. While exploring a strange city, they are captured by Daleks. When it is determined that the party is suffering from radiation poisoning, Susan is allowed to return to the TARDIS to gather some drugs that will cure them. The Daleks intend for her to lead the Thals (a 'mutated' race) into a trap.
After returning with drugs provided by the Thals, Susan, The Doctor and his companions escape from the city only to find that the Daleks had removed the perfectly good fluid link from the Doctor before they were imprisoned. The TARDIS crew must help the Thals take the Dalek city in order to save themselves from a neutron bomb and to regain the fluid link.