Maria moves into her new home and meets her neighbour Sarah Jane Smith. It isn't long until both of them find themselves at the local Bubble Shock factory and in terrible danger.

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.