Maria Jackson wakes up one morning to find that seemingly no one has heard of Sarah Jane or Luke Smith. Maria discovers that a woman called Andrea Yates has taken Sarah Jane's place.
Investigating, Maria finds a 1964 newspaper report stating that a thirteen-year-old Sarah Jane Smith drowned after falling from a pier, where she was playing with her friend Andrea Yates. As Maria watches, the names of the deceased and the survivor keep switching. When Maria confronts Andrea, the panic stricken woman rushes to her attic and finds a second puzzle box. A cloaked figure appears and offers to make Maria disappear. After Andrea accepts, he dispatches a Graske, who captures Maria just after Alan picks up the first puzzle box. When Alan comes round he discovers his ex-wife cannot remember their daughter. Meanwhile Maria escapes from the Graske and finds herself on a beach promenade near two girls. They introduce themselves as Andrea Yates and Sarah Jane Smith.
After failing to dissuade Andrea from taking Sarah Jane to the pier, Maria is recaptured and taken to join Sarah Jane on a white misty plain, limbo. The mysterious figure explains he has removed Sarah Jane from Earth's timeline so the meteorite will destroy it and create the chaos on which he feeds; the Doctor will be his next target.
Alan accompanies Chrissie to Andrea's birthday party. Remembering Maria's suspicions, he questions Andrea, who takes him to the attic and tearfully reveals the truth. When she fell off the pier and Sarah Jane was unable to save her, a voice offered to switch the girls' places, and she accepted. The figure appeared and gave her the puzzle box, then removed himself from Andrea's memory; Maria’s questions made Andrea remember. Alan is now chased into the street by the Graske but knocks it down and ties it up, then uses its device to bring back Maria.
They return to the attic, where Sarah Jane appears in the mirror and explains to Andrea that witnessing her death gave her the resolve to fight pointless deaths herself. When the figure reappears (called “the Trickster” by Alan), Andrea explains she has changed her mind about the deal and throws her puzzle box at the mirror, smashing both. Back in 1964 Andrea falls to her death. Sarah Jane and Luke reappear in the attic and activate Mr Smith. The party guests, including Clyde and Chrissie, have learned from television news about the incoming meteorite, and are relieved to see it suddenly diverted from its fall. The episode ends with Alan demanding an explanation of his daughter’s involvement with aliens and supercomputers.