Sarah Jane and her companions investigate claims of sightings of a ghostly nun at Lavender Lawns, the local nursing home. Meanwhile, Chrissie Jackson moves into her ex-husband's house, but succeeds only in causing further problems with the family. Back at Lavender Lawns, an old lady gives Luke an ancient talisman, which is really the key to a portal in space and time. They find that a group of nuns are hiding an age-old creature, the Gorgon. When Sarah Jane refuses to give the talisman to the nuns, they kidnap Luke and Clyde and take the Gorgon and Maria to Sarah Jane's house where the Gorgon turns Maria's father to stone.
Having got what they came for, the nuns and Gorgon leave. Luke and Clyde escape from their entrapment via a secret passage. Mr Smith tells Maria and Sarah jane that Alan is retrievable until 16:00. Luke and Clyde see the Gorgon stumble, but are unable to prevent the talisman beginning the process of joining the Gorgon Homeworld with Earth. The Gorgon - a parasite inside the Abbess - chooses Sarah Jane as its next host. Chatting with Bea reveals to Maria that the talisman can revert those turned to stone. Clyde distracts the nuns long enough that Luke grabs the talisman, disconnecting the portal. However, Sarah Jane and both boys are recaptured and locked in another room, though Sarah Jane is soon taken and tied next to the portal. Luke and Clyde escape again and join Sarah Jane just as the Gorgon begins to transfer itself, but Maria arrives and uses a mirror to revert the transfer and turn the Gorgon - and Abbess - to stone, freeing the nuns of mind-control. Maria disconnects the talisman and the portal shuts down forever. The talisman brings Alan back to flesh-and-blood and Chrissie leaves, reuniting with Ivan.