Maria and Luke start their new school where they pal up with another boy, Clyde. However, two of the teachers and the class genius turn out to be Slitheens in human form. They are shapeless smelly invaders from another planet whose aim is to destroy the Earth by turning off the Sun with the aid of a machine which Luke inadvertently helps them to start. The three teenagers are chased around the school by the Slitheen family.

The kids escape from the Slitheen and meet up with Sarah Jane at her house. They discover that vinegar will kill the Slitheen so, armed with vinegar bottles, they return to the school and destroy the three adult Slitheen and their machine. Carl, who took on the form of the class brain-box, escapes via a teleport but Sarah Jane assumes that he is dead. Back home she accepts Clyde into the group but asks for him not to tell others about their experiences.

Sarah Jane (Elisabeth Sladen) and Maria (Yasmine Paige) investigate the mysterious disappearance of local boy Lance Metcalf, and soon discover that a number of children have gone missing across the country. Remarkably, the disappearances all occurred during freak storms and in cities that are home to a laser tag game called Combat 3000. As Sarah Jane and Maria delve deeper into the mystery, Luke (Tommy Knight) and Clyde (David Anthony) are off enjoying a game of Combat 30…

A Carcaroth has arrived on Earth, seeking soldiers to help fight his endless intergalactic war back home. Using Combat 3000 to sift out the best, most aggressive and instinctive fighters, he then transports them to his spaceship – awaiting transfer to the war. Clyde (David Anthony) and Luke (Tommy Knight) find themselves amongst the 'warriors', while Sarah Jane (Elisabeth Sladen) and Maria (Yasmine Paige) must infiltrate the alien's headquarters to bring down the Carcaroth from the inside.