The Second Doctor and Jamie are sent by the Time Lords to investigate a research facility conducting unauthorised time experiments. On their arrival, they discover a familiar and treacherous enemy. The Sixth Doctor and Peri arrive to find a shell-shocked Jamie claiming that the Doctor has been killed. It is in fact all part of a Sontaran plot to discover the secrets of time travel, which necessitates a trip to Seville to rescue the earlier incarnation of the Time Lord.