Gita is kidnapped from her florists late one night by Mrs Wormwood, who leaves a cheque with her name on it. In the morning, Rani calls upon Sarah Jane to help her find her mother. Upon investigating the flower shop, they discover the cheque and take it back to Mr Smith for analysis. Mr Smith's scan reveals that the sort code and account number are fake, but are actually a code. When translated, they give an address of an abandoned warehouse. Rani and Sarah Jane find that the door is open and find Gita in a trance. Mrs Wormwood enters and remarks on Rani being taller than Maria. Mrs Wormwood tells Sarah Jane she needs her help. She says that since the Bane's plan to take over the Earth failed, she has been exiled and is being hunted. The Bane attacks, but they manage to escape.
Sarah Jane takes Mrs. Wormwood back to her house and Mr. Smith locks her in a forcefield. Mrs. Wormwood says that an ancient immortal, Horath, tried to take over the galaxy, but was defeated three thousand years previously. Unable to destroy him, Horath's body and consciousness were separated and placed at opposite ends of the galaxy. Mrs. Wormwood says that the Bane have discovered Horath's consciousness, and are trying to find his body, which is located on Earth. Once united, she says the Bane will be able to conquer the galaxy. The only clue to the body's location is in the Tunguska Scroll, which is kept in UNIT's Black Archive, a secure storehouse of alien artifacts.
Sarah Jane visits her old friend, retired Brigadier Sir Alistair Gordon Lethbridge-Stewart, to get his help to enter the Archive. Sarah Jane and Rani obtain the Tunguska Scroll, which has information on the location of the body of Horath. UNIT personnel stop her, but she uses her sonic lipstick and escapes with Rani and the Brigadier. Meanwhile Luke talks to Mrs. Wormwood and when the Bane attack the house, he releases her from the forcefield. She is about to escape when a Bane tries to eat her. Sontaran Commander Kaagh shoots it and reveals himself to be her agent.
Mrs Wormwood saves Luke, trying to tell him something, then Clyde and Sarah Jane arrive. Sarah Jane, Luke, Clyde, Rani, the Brigadier and Mrs Wormwood all hide in Gita's Flower shop so that UNIT cannot find them. Mrs Wormwood asks Sarah Jane for the Scroll but she refuses. Commander Kaagh arrives and forces Sarah Jane to give it up by threatening Luke, Clyde and Rani. With the scroll in hand, Mrs Wormwood forces Luke to go with her and Commander Kaagh. Mrs Wormwood, Kaagh and Luke arrive at a factory where Kaagh has hidden his space pod. He places the Consciousness of Horath in a scroll. Mrs Wormwood says to Luke that he could be a ruler of a galaxy as her son. Meanwhile the others go to Sarah Jane's House to speak to Mr Smith, to try to find the whereabouts of Luke, Mrs Wormwood and Kaagh. Once they find out, they borrow Gita's van and go after them. Mrs Wormwood, Luke and Kaagh arrive at a stone circle where they force Luke to insert the scroll. Sarah Jane arrives. Kaagh betrays Mrs Wormwood and he stops her by jumping into the portal and dragging her in with him.