Luke Smith is going for a sleepover at Clyde Langer's house whilst Sarah Jane goes away for the weekend. Clyde's estranged father Paul turns up hoping to get to know his son. Jacob, the unpopular kid in school, is in possession of an alien pendant. Jacob discovers that he can use it to control other people. When he takes a teacher's and other student's voice in a detention, Jacob rushes to the bathroom to wash the handmark off. When he successfully does so, Rani knocks on the door and distracts him. Jacob leaves the room but hides outside. Rani enters the bathroom cautiously, and finds the time-watch. When she discovers, through instructing her dad, that it can control people, she takes it to Sarah Jane's house for Mr Smith to investigate it. Sarah Jane, however, has disabled Mr Smith while she is away. Rani leaves the pendant hanging on a roof beam. Clyde tells his father about the adventures he has with Luke, "saving the world". When his father does not believe him, Clyde takes him to visit Sarah Jane's attic. While Clyde shows him some of the objects in the attic and tells him about Mr Smith, Paul pockets the pendant. When they leave the house, Rani's father spots them and comes over to investigate. In possession of the alien pendant, Paul discovers that he can control people, after making Rani's father a victim of the pendant for the second time that day. After Luke and Rani turn up, Paul instructs his son Clyde, over whom he now has control, to forget who they are and that he has ever seen them before. Clyde, as he must, accepts his father's command.
Luke and Rani send a picture of the pendant to Maria and her father to investigate as her father can hack into UNIT. They discover that the pendant belongs to a race of beings called Berserkers, and that using the pendant can change the user into one of them, including marking their hands and turning their veins blue. Rani's father refuses to stop the pressups. Paul gets everything he wants for free, and uses this as a bonding method with his son. He instructs Clyde to forget about how he left him and his mum when he was young and, indeed, to forget about his mum. The effects of the pendant take over Paul's skin and his mind. He cannot control it and collapses, but then stands up, now possessed by the pendant. He announces that he is the Berserker and that Clyde is his soldier. Sarah Jane appears and Clyde recognizes her. She tells him that his mum, Rani, and Luke are the most important people in his life when he asks who they are. Clyde and his mum talk Paul through his memories, whilst Sarah Jane shows him a reflection of himself in the mirror, and he remembers who he really is. Everyone who the Berserker commanded has their commands undone. Clyde tries to get his mum and dad back together, but Paul tells his son that he has made his Mum's sister pregnant. Clyde tells his father he doesn't need him and not to mess up with this baby. Clyde tells his mother to forget about the pendant, what she knows about Sarah Jane's adventures with her son, and his father. Clyde chucks away the pendant, into the sea. Sarah Jane reveals that her parents died when she was young and she would do anything to see them again. When Clyde is gone, she takes out a picture of her mother and father and rubs her finger over the latter.