Alan Jackson is offered a job in Washington, D.C. in the United States of America, but seeks daughter Maria's and ex-wife Chrissie's approval before he relocates himself and Maria there.
After strange lights are sighted around the Tycho Radio Tower, Sarah Jane Smith, Luke Smith, Clyde Langer and Maria investigate. As shown in the picture, they discover Sontaran Commander Kaagh, the only survivor of the Tenth Sontaran Battle Fleet which was otherwise seen to be destroyed in Doctor Who episode "The Poison Sky". Kaagh plans to avenge his fleet by bringing Earth's satellites down on nuclear power plants across the world thereby wiping out all of humanity with the resultant explosions.
Failing to overpower Kaagh as he implements his plan, Sarah Jane, Luke, Clyde and Maria are saved by the arrival of Alan and Chrissie, the latter discovering the truth about Sarah Jane's alien investigations. Chrissie knocks Kaagh unconscious by jamming the high-heel of her shoe into the probic vent on the back of his neck, but an electrical charge also knocks her out and Alan and Maria think they can pass the day's events off to her as a dream. Luke is able to deactivate the computer Kaagh has rigged to ground the satellites and with his ship's weapons deactivated, Kaagh is sent back to his home planet, Sontar.
Six weeks later, Alan and Maria leave for America. Chrissie reveals to Sarah Jane that she remembers everything about the Sontaran incident.