The Eternity Trap

Professor Rivers tells the story of Ashen Hill Manor, which is said to be haunted. She talks of a man named Lord Marchwood and how he employed Erasmus Darkening. Lord Marchwood thought that Erasmus could make gold out of base metals. His two children, Elizabeth and Joseph, spy on Erasmus. He spots them and makes them vanish. Professor Rivers is telling the story to Sarah Jane, Clyde and Rani. She and her assistant Toby are making a scientific investigation of the ghostly phenomena. Luke is at home as he has no interest in ghost-hunting. Sarah Jane splits up from the group and finds a book on the history of the manor. She turns round from the book case which she is reading from and unknown to her, the books begin moving. Clyde and Rani go outside and find a dry fountain. All of a sudden the fountain starts and some wet foot prints appear leading in the direction of a shed. Clyde and Rani hear a young girl crying, and enter the shed. The crying stops as Clyde pulls a sheet off a mirror. A strange man's face appears in the mirror. Clyde and Rani run back into the manor and find Sarah Jane. Meanwhile, in the control room the camera in the playroom cuts out. Professor Rivers goes to investigate. The baby monitor starts to play, then seconds later the professor vanishes. They all go to investigate and they find that all the toys have started to move on their own. On the mirror in the room a message appears saying "GET OUT". Clyde and Rani begin exploring and find a secret passageway; they go down and find Erasmus' lab. Meanwhile, Sarah Jane goes out onto a balcony. A creature in the bushes with red eyes appears and advances towards Sarah Jane. Lord Marchwood's ghost manifests and fights off the creature. Clyde and Rani find a computer, then when they turn around they see Erasmus Darkening.

Lord Marchwood arrives to rescue Clyde and Rani. Then they are aided by his children. Sarah Jane hears a recording of Professor Rivers saying "Sarah Jane help me" over and over. Then she sees her. She warns her that "he is coming for you" and then disappears. Sarah Jane reunites with Clyde and Rani and then they find Lord Marchwood. Sarah Jane believes they are not ghosts. Toby, Sarah Jane, Clyde and Rani discover all the people that have disappeared except for Professor Rivers who has not been absorbed yet. Erasmus arrives and they all disappear. Erasmus tells them he will come for them all. Sarah Jane knows Erasmus is playing with them. Sarah Jane and Toby investigate Erasmus' secret chamber while Rani and Clyde distract Erasmus. Toby tells Sarah Jane about a creature who came into his room and watched him every night when he was younger. Rani and Clyde are chased by something into a game room. When all the games start playing themselves they run outside and get locked out. Sarah Jane and Toby find Erasmus' machine. Sarah Jane suspects that Erasmus was trying to get home to another galaxy with the machine. The machine malfunctioned and all the people that disappeared got trapped between dimensions. Clyde and Rani run into the pavilion. Sarah Jane and Toby get interrupted by Erasmus. Sarah Jane works out that the machine is giving Erasmus and all the people that disappeared immortality. She also works out the creature that attacked her earlier came through the machine. Lord Marchwood once again rescues Clyde and Rani again by slaying the creature. Sarah Jane, Clyde, Rani and Toby reunite along with Lord Marchwood. Sarah Jane comes up with a plan to stop Erasmus. They can turn him into electricity but he has to step into a certain area. Lord Marchwood uses his sword and a wire to connect to it to do it. Erasmus is destroyed and Professor Rivers is returned. Sarah Jane destroys Erasmus' machine with the sonic lipstick. Sarah Jane, Clyde and Rani say goodbye to Professor Rivers and Toby. Sarah Jane says that she still doesn't believe in ghosts. They drive away. We then see Lord Marchwood and his children at the window.

• Tommy Knight was not available for filming of this episode due to his GCSE exams. This story is therefore the first not to feature Luke Smith. As a result of this absence, Elisabeth Sladen becomes the only cast member to have appeared in every episode of the series. Other series mainstays such as Mr Smith, K9, Sarah Jane's attic and Rani's parents, Haresh and Gita, do not appear in this story.
• Professor Celeste Rivers and the Pharos Institute were previously shown in The Lost Boy and The Day of the Clown.
• This is the first episode with scenes set prior to Sarah Jane's birth.