A child, Oscar, from the 1950s walks into a time fissure and is transported to the current year. Unbeknown to Sarah Jane, who takes him back, he did it deliberately under the command of the Trickster. Having spotted a milestone, she later asks Mr Smith about the village she could see in the distance, discovering it was Foxgrove, the village where she was born. When Luke inquires about her odd behaviour, she shows him a photograph of her parents and tells him of how they abandoned her as a baby by the side of the road before being killed in a car accident, leaving her to be looked after by her aunt.
Though realising that the time fissure was a trap and initially refusing herself the opportunity to go back and meet her parents, that evening she does so, but accompanied by Luke who had anticipated her actions. Rani and Clyde ask Mr Smith where they have gone, and realise that she has gone back to when her parents were alive. Then the box from the episode Whatever Happened to Sarah Jane?, the box that Maria had lights up. Sarah Jane and Luke spot her mother at the village fete carrying Sarah Jane as a baby. When they shake hands with Sarah's father, they introduce themselves as David and Victoria Beckham. Luke spots the child they 'helped' earlier and follows him. Sarah Jane has a conversation with her mother, who reveals her plan is for Sarah Jane to have siblings. Sarah Jane leaves before she is forced to hold herself in baby form, which would have created a paradox. On seeing a newspaper, Sarah Jane realizes that the day she has visited is the day her parents died. She ignores Luke's warning that changing time was a trap, and sabotages her father's car thereby preventing the accident. The child, Oscar, reveals that he is in fact a Graske. Sarah Jane and Luke go through the time fissure again and discover that present-day London has been destroyed. The Trickster then appears before Sarah Jane and mockingly thanks her for all she has done for him.
Sarah Jane demands to know how saving her parents could have altered time so drastically, to which the Trickster explains the village was built on a fault in time. Sarah Jane's tampering with a fixed point in time opened this fault, allowing the Trickster to physically enter their reality and ravage Earth for the last half century. Sarah Jane and Luke go back through the fissure as the Trickster gloats he has already won.
Meanwhile, Clyde and Rani explore the ruined London, reduced to rock formations and worked by the last surviving human slaves who are led by the Trickster's Graske. The slaves are being made to mine resources so that the Trickster may create a ship and extend his reach to the rest of the universe. Clyde and Rani implore the Graske to help them restore the original timeline but he explains that if he did the Trickster would punish him; years ago the Graske had nearly died in space, but had been saved by the Trickster in return for eternal servitude. Clyde promises the Graske that if he helps them, they will give him the puzzle box to free him from the Trickster. The Graske agrees and reopens the fissure so that Rani can tell Sarah Jane the new information.
In the past, Sarah Jane's parents come to realise who she is and what she was trying to prevent. However, their touch now instantly withers any organic life they hold and they realise it is their destiny to drive off this day. After telling their daughter how proud they are of her, they both enter the new repaired car and drive off. The Trickster begins to fade, unable to see how the timeline could have been restored. Sarah Jane proudly states that the Trickster's plan never considered that her parents would willingly sacrifice themselves to save the world. The Trickster vanishes, howling in pain.
Back in the modern day, the timeline is restored around the Graske and Clyde, who gives the alien the box. The Graske teleports away, beaming over its newly returned freedom. The fissure reopens as Sarah Jane, Luke and Rani return.
Later at home, Sarah Jane reminisces over her parents. Although she could not save them, she finally knows why they left and is incredibly proud of them. Mysteriously, the camera pans to an old piece of paper reading "Mr Smith, I Need You" on Sarah Jane's desk.