Maria and Luke start their new school but find all is not as it seems. Aided by Sarah Jane and their new friend Clyde, they find the Slitheen, old enemies of the Doctor, this time disguised as teachers, have taken control of the technology block and are trying to switch off the sun. Luke unknowingly gives the Slitheen the code to start the machine, and they start to absorb the power of the sun. Sarah Jane investigates the company controlling the science block, and Maria, Luke and Clyde investigate the school. Sarah Jane is attacked by a female Slitheen who disguised herself as a company secretary, Janine, Luke finds a secret room and is confronted by the Slitheen commander, his headmaster Blakeman, whilst Maria and Clyde are chased first by their Slitheen science teacher, Mr. Jeffrey and then the school genius, Carl, unzips in front of them to reveal a child Slitheen.
They all manage to escape and head back to Sarah Jane's house. Clyde figures out that the Slitheen are negatively affected by vinegar, and the team head off back to the school with squirty bottles of vinegar. After Maria uses the vinegar to blow up the Slitheen commander Blakeman, they head over to the secret room in the block and Luke tricks the Slitheen by telling them the machine won't take the power of the sun. He turns the machine off and uses the Sonic Lipstick to cause the machine to malfunction. As the room explodes, the continental Slitheen, still disguised, escape whilst Janine is electrocuted and explodes and Jeffery and Carl are trapped. They beg for Sarah's help, and she tries to open the door, but Jeffery is killed when the technology explodes. However, at the same time, Carl escapes using a teleport. Sarah Jane is highly affected by this, as she is unaware of the teleport and thinks Carl is dead with his dad. Afterwards, she tells Clyde about her travels with the Doctor, and then accepts him into the team, before sharing a long hug with Luke.