Alan has a dilemma as he has been offered a job in America and feels guilty about taking Maria away from her mother and friends. Maria, however, is busy helping Sarah Jane, Luke and Clyde investigate strange lights coming from the Tycho radio tower. They discover that Kaagh, the last survivor of an invasion of Earth by the Sontaran race, is planning his revenge and has taken over Professor Skinner and his computer,the plan being to bring down Earth's satellites onto nuclear power stations worldwide and cause an explosion. The professor's daughter helps them to escape the building and get out into the woods.