One night, a High Priest and Priestess of a coven, wearing goat masks light candles, beginning a ceremony. The other coven members join hands and dance around the altar chanting “Hecate! Hecate! Hecate!

Sarah Jane Smith's Aunt Lavinia is called away early to a lecture tour in America, and leaves her market garden in the hands of her partner Commander Bill Pollock and farmer George Tracey. She leaves under a bit of a cloud, as the villagers of Morton Harewood have taken exception to the letters she recently wrote to the local paper, denouncing their belief in witchcraft and claiming that it is still actively practiced in the area. Unable to contact Sarah before leaving, Lavinia tells her friend Juno Baker to expect Sarah's arrival; Sarah will be spending the holiday here with Lavinia's new ward, young Brendan Richards. And perhaps she'll finally find time to open the mysterious crate which Lavinia has been keeping in the attic in Croydon for the last three years.

Sarah arrives a few days before Christmas only to be greeted by the surly George Tracey, and is surprised not to have any word from Lavinia -- it isn't like her aunt to go without any word. Brendan calls from the station, having taken the train as he was fed up waiting for her at his school. She picks him up and brings him back to the house, where they meet Commander Pollock. Juno Baker calls to invite Sarah to a get-together with other villagers, but Pollock advises her not to go as the Bakers are her aunt's biggest rivals in the market garden business. Once Pollock leaves, Sarah and Brendan set to the mysterious crate, which when opened reveals what appears to be a mechanical dog. Sarah accidentally activates it when she leans on its control panel, and it introduces itself as K-9, Mark III -- a gift from her old friend the Doctor...

Brendan is utterly enchanted by his new acquaintance, and Sarah leaves him to put K-9 through his paces while she sets off to find word of her aunt. The local postmistress, Lily Gregson, is no help, claiming that her aunt never sent any cables and warning Sarah that many locals were upset by her letters to the paper. Sarah decides to go to the Bakers' party after all, where Juno confirms Lily's claims...

Brendan is using K-9 to test soil samples from the garden when he is attacked by George Tracey and Tracey's son Peter. K-9 stuns Peter with a blast from his nose laser and George flees in terror before Brendan can get a good look at him. As K-9 pursues him, Peter recovers and desperately warns Berndan to get away before it's too late. Searching for the intrduer in the garden, K-9 accidentally knocks over a set of ladders into the glassed-over growing area, causing several hundred quid's worth of damage. George takes the opportunity to flee, and when Brendan comes out to investigate the noise Peter manages to slip his bonds and escape as well. George is trying to convince the High Priest of the local coven that his son was taken from him by Hecate's familiar, a great white dog, when Peter arrives -- and as he hasn't seen K-9 he's unable to corroborate his father's claim. Brendan and K-9 tell their story to Sarah when she returns, but she decides to report it to the police later.

The next morning, Brendan and Sarah find Pollock and Tracey studying the damage to the garden, and when Brendan tries to show off by citing the pH of the soil which K-9 analysed last night, Tracey snaps at him, demonstrating his experience and love of the land. Pollock reminds Brendan that science can't control the elements yet; people here are still at the mercy of the weather. That night, George sends his terrified son back to the manor to kidnap Brendan again, and Sarah, studying her aunt's books on witchcraft, doesn't realize until morning that Brendan is gone and the phone lines have been cut...

Convinced that Tracey knows more than he's saying, Sarah smuggles K-9 into his cottage before going with Pollock to report the incident to the police. The police prove unhelpful, but Staff Sergeant Wilson tells Sarah that Peter Tracey has also vanished -- having been placed in the custody of his father some time ago as a suspended sentence after housebreaking. But as soon as Sarah and Pollock have gone, Wilson heads for Tracey's house to warn him of the danger. K-9, hidden under Tracey's stairwell, listens in as the conversation makes it clear that Wilson and Tracey are part of a witches' coven which intends to murder Brendan as a human sacrifice. Tracey is convinced that Hecate will punish him for disobedience, but Wilson can't go along with human sacrifice and sets off to warn somebody.

Tracey departs his cottage soon afterwards, and Sarah collects K-9 from the cottage, and is horrified by his revelations. She immediately sets off to find Wilson only to find his dead body in the road nearby; somebody has tied up a goat on his bike path and he has suffered a heart attack from fright. She tries to explain the situation to Pollock, who finds it difficult to believe but agrees to help her. Later that night, the reluctant Peter Tracey is finally initiated into the coven...

The next day, Sarah goes to meet Pollock only to find that he too has vanished from his home. The police don't believe her story, and hampered by her inability to explain K-9, she has claimed to have overheard Wilson and Tracey's conversation herself -- but can't explain how she did so while hiding outside the house. She goes to the Bakers for help but finds them perhaps just a little too eager to dismiss their claims. Back at the manor, K-9 studies Lavinia's books on witchcraft and announces that the sacrifice will take place at midnight on the winter solstice, December 22 -- and that's just in a few hours' time.

Juno calls Sarah to invite her over, claiming she shouldn't be left alone while she's so clearly distraught, but Sarah turns down the invitation and sets off with K-9 to search all churches, hallowed ground, within a five mile radius. Although somebody tries to kill her by running her off the road with a driverless tractor, she only becomes more determined... and yet she and K-9 come up empty after checking all churches within the area. With less than an hour to go until midnight, K-9 -- unaware until now of its significance -- points out that the map of the area indicates that there is an abandoned chapel on the grounds of the Manor itself. Sarah and K-9 rush back to the manor and arrive at the chapel with seconds to spare; just as the High Priest and Priestess of the coven are about to stab the drugged Brendan with a sacrificial knife, K-9 and Sarah arrive, and K-9 blasts down both the Priest and Priestess, throwing the locals into utter disarray. As the would-be witches scatter in terror, Sarah removes the goat masks from the High Priest and Priestess... to reveal that they are Bill Pollock and Lily Gregson.