The next day Sarah, Luke and Clyde arrive at the home to visit the woman, Mrs Randall, a friend of Clyde’s grandmother. As she and Sarah discuss the nun, who has been seen by other residents in the past, Luke finds another woman outside, beckoning to him.
Back in Bannerman Road Maria’s mother has come to visit, hoping to make her new boyfriend jealous be temporarily leaving him. Although her ex husband is not impressed, Maria is far more jubilant at having her mother living with her again.
At the rest home Luke joins the woman, Mrs Nelson-Stanley outside, where he discovers she believes herself to be in Egypt waiting for her husband. However, she soon snaps out of her delusion, recognising Luke as the boy from the window, and after revealing she too has seen the nun, leads Luke away.
Inside Sarah talks with the owner of the home, Mrs Gribbins, who is far more sceptical of the story of the nun, thinking Sarah is chasing nothing but superstition in her search for a story.
In the grounds Luke is led to an ivy-covered plinth which the old woman, who reaches into the foliage and pulls out a battered tin box. Inside she reveals an ornate talisman, which she hands over to him to keep, with only one instruction: “You must not let her get it”. Luke takes the item; unaware Mrs Gribbins is watching them from the window.
In Maria’s room her mother enters, offering to take her daughter shopping. However, Maria has homework to do and the two are soon arguing, Maria distressed by her mother’s attitude to Sarah Jane and Luke, as well as her sudden appearance back into her and her father’s lives.
Clyde, Sarah and Luke leave Lavender Lawns, Clyde moaning about the elderly whilst Sarah explains to her young friends that everyone must get old. Clyde protests, claiming that when he is older the technology will be in place for his brain to be fused in a robotic body. They drive away, again under the watchful eye of Mrs Gribbins.
Maria’s dad consoles her over the previous argument, but she is still bitter about her mother’s negative attitude toward their neighbours. She then begins to turn on him, upset he seems to have moved on quicker than she has following his divorce, her life is changing and she isn’t prepared for it.
Meanwhile, at a nearby abbey, Mrs Gribbins arrives and knocks upon the door. When it is answered she asks to see Sister Helena.
In her attic, Sarah calls for Mr Smith and sure enough the giant computer appears from within one of the walls. Lavender. He begins to research the history of the Lavender Lawns Rest Home, but he finds nothing peculiar. He then detects the object Luke has been given and identifies it as alien. He is unsure if it is safe and as Sarah asks Luke where he acquired it, a moody Maria enters. Sarah decides she must talk with Mrs Nelson-Stanley, and she takes the young girl along with her leaving the boys in the house.
At Maria’s house, her mother is annoyed by her daughter’s row with her. She finds her ex-husband somewhat unsympathetic, he is disgruntled by the fact she seems not to notice how she affects her daughter by turning up only when it suits her.
At the Abbey Mrs Gribbins explains about the talisman and how it has been given to Luke. Sister Helena is annoyed that matters have now been complicated; the Abbess will want to see her. She is taken away to a room in which an elderly woman is seated in a chair. She raises her withered hands and removes her veil, causing Mrs Gribbins to scream.
At Lavender Lawns, Mrs Randall takes Sarah and Maria to Mrs Nelson-Stanley’s room, where they discover her husband was an archaeologist. He died some years ago and since then she’s gone apparently mad. Maria sees the talisman in a photo of the pair. Mrs Norman-Stanley soon arrives and Sarah asks where she got the talisman. She does not answer, and instead recalls her husband’s opinions of a race known as the Sontarans. Mr Randall explains the old woman talks at length about the Gorgon, a creature from old horror films.
Sister Helena arrives at Sarah’s house, looking for talisman. Luke reveals that he has it, but Clyde refuses to hand it over. Maria’s dad arrives and she lies to him about visiting for a donation to repair the Abbey roof, before leaving hastily. Clyde and Luke then explain Maria has gone with Sarah but will return soon. Clyde tries to explain to Luke about discretion before deciding they should return to the rest home to explain about the sister, unaware a hearse is following them.
Sarah and Maria realise Bea is not talking gibberish; Sarah has also encountered creatures known as the Sontarans. Mrs Nelson-Stanley digs out an old record whilst Sarah questions her about the talisman. She explains it was unearthed in Syria by her husband, and insists, “They mustn’t find it.” The sisters are looking for it, and they protect the Gorgon.
The two friends then return home, where Sarah unearths a book of Greek mythology. She explains that there were three Gorgons, one of whom was Medusa, a creature with serpents to hair who turned her enemies into stone with a single glance. She theorises that if Mrs Nelson-Stanley is right, then one of the Gorgons may still be alive.
Sister Helena pulls up in her hearse and meets Luke and Clyde at the roadside, promising not to harm them. She claims she wants to talk to them, and explains that the talisman is dangerous, “more than you can imagine”. She grabs Luke and puts him in the car with the intent of taking him to Abbey, leaving Clyde alone on the pavement.
Sarah explains to Maria that the Gorgon might be an alien, still alive since time of the Greeks. Clyde phones and explains what has happened to Luke. Soon he arrives back at the house and he, Sarah and Luke travel to St Agnes Abbey. Sarah bluffs her way inside with a fake identity, whilst Clyde and Maria sneak off around the back.
Sarah arrives in a library and is locked inside by one of the nuns. Meanwhile, Maria and Clyde sneak around the back of the Abbey and try to get inside, finding an open window through which to enter. They begin looking for Luke; worried they might find the Gorgon instead. They enter one of the rooms, where they find a statue of Mrs Gribbins. Suddenly the door opens and the nuns enter; they have been caught.
Luke is brought into the library, followed by Helena. As Sarah rejoices at having her son back, the Sister claims the kidnap was only to get her attention. Clyde and Maria are then brought in and they explain about Mrs Gribbins. Helena then explains that although the image of Medusa was a Greek embellishment, the creature was indeed one of the Gorgons, who now survives as the withered Abbess of the Abbey, whom generations of the Sisterhood have sworn to protect.
She tells them that of the three original Gorgons, only one remains. One was killed in the ancient days of Greece, when the key that helped the Gorgons come to Earth was stolen. Since then the Sisterhood have searched for it, only coming close some fifty years ago when Mrs Nelson-Stanley and her husband uncovered it. Sarah deduces that it was then that the second Gorgon died.
Helena then claims that the Gorgon just wants to go home to die, and Sarah reluctantly agrees to help her do so by handing over the talisman. She and Maria return home, whilst the nuns keep the boys at the abbey as insurance. The Gorgon is taken with them, accompanied by Helena and some of the other Sisters. Sarah returns to the attic and retrieves the talisman. She arms herself with her Sonic Lipstick then goes back downstairs.
Maria questions Helena about the Gorgon but she receives no reply. Sarah enters with key to the portal but threatens to destroy it unless the boys are let go. Helena warns that she has gone too far and the Gorgon is brought before her. She warns Maria not to look at its face and they turn away. The creature reveals itself to them, a withered humanoid figure with glowing eyes and a beam f light emitting from her mouth.
Suddenly Maria’s father enters but it is too late for him to protect himself. The Gorgon screams and he is turned to stone.