Another adventure for everyone's favourite Time Lord. The TARDIS is forced to land on the planet Exxilon after suffering a sudden power loss. The Doctor (Jon Pertwee) and Sarah encounter a similarly stranded expedition from Earth, who are seeking a cure for the plague which is spreading rapidly across the galaxy. The Doctor believes that he can find a solution in the technologically advanced but long-abandoned city of the Exxilons, but becomes caught up in a power struggle between the humans and the latest arrivals to the planet: his old enemies, the Daleks.