Plot
Sarah Jane Smith visits her Aunt Lavinia, who was occasionally mentioned but never seen in Doctor Who. When she arrives at her aunt's house, though, she finds that her learned relative has left early for a lecture tour in America, Christmas notwithstanding. Sarah is thus left disappointed by the prospect of another holiday without family. Lavinia's ward, Brendan Richards, breaks her moment of reflection on her aunt's sudden disappearance. After picking him up from the railway station, they return to the house and discover a large crate that has been waiting for Sarah for a number of years. When they open it, they discover a mechanical dog named K9. Upon activation, it tells Sarah that it is a gift from the Doctor.
Brendan's curiosity about K9 is matched only by Sarah's renewed concern over Lavinia's absence. They thus split up and follow their new-found obsessions. Sarah goes into town to question the locals, and Brendan stays behind to test the capabilities of Sarah's new "pet". In town, Sarah discovers that Lavinia has become disliked by some because of her blunt letters to the local newspaper editors about a growing practice of witchcraft in the area. Brendan, meanwhile, is attacked while using K9 to analyse soil samples in Lavinia's garden. His attackers, George Tracey and his son, Peter, are tied into the local coven. George Tracey flees before Brendan can get a good look at him, however K9 uses his laser gun to stun Peter before setting off in pursuit of George. Peter is pinioned and interrogated by Brendan, but makes his escape when Brendan goes outside to investigate a crashing sound which turns out to be the accidental destruction of a greenhouse by K9 in his pursuit of the elder Tracey.
Since Tracey is actually Lavinia's gardener, he is naturally called in the next morning to investigate the damage K9's pursuit of him caused to the greenhouse. After Brendan attempts to brag about the pH balance of the soil, Tracey sharply comments that gardening is more about respect for nature than scientific theory. Otherwise, though, he doesn't betray his more sinister intent towards Brendan. Later that night, he sends his son out to kidnap the sleeping Brendan from the house.
This time, Brendan's attacker is successful, stealing him out from under Sarah, who is elsewhere in the house, reading up on the local practice of witchcraft.
Sarah is now increasingly suspicious of Tracey, believing he would have the opportunity to commit the crime, even if she can't yet put her finger on the motive. She therefore finds a way to hide K9 in Tracey's house. K9 quietly monitors the household, until he eventually listens in on a conversation that implicates Tracey as a member of a coven. He also discovers that Tracey intends to kill Brendan in an act of ritual murder.
When Tracey leaves his cottage, Sarah is able to retrieve K9, who alerts his new mistress to the impending crime. She has no way to enlist the aid of the local police or, really anyone else in the town, because she can't substantiate her claim of overhearing the conversation without also then having to explain who and what the anachronistic K9 actually is.
Realising that she and K9 are effectively on their own, she tries to figure out how to stop the sacrifice. Her first order of business is determining the when of it. Using Lavinia's books on witchcraft, she and K9 deduce it must occur at midnight on the winter solstice, now just a few short hours away. The where of it is more elusive, however, causing the duo to drive around the shire looking at all the churches. As the last few minutes before midnight tick away, they finally realise that there's an abandoned chapel on Lavinia's property. Rushing home, K9 and Sarah are briefly upset at missing something that was right under their noses all along.
They arrive just in time for K9 to use his blaster to stop the coven's Priest and Priestess from plunging a knife into Brendan's chest. Now stunned, the group's ringleaders are easily apprehended by the police.
Finally able to celebrate Christmas, Sarah receives a call from her Aunt Lavinia. She's surprised that Sarah was worried about her, since she left instructions for her business partner to send Sarah a cable. As he turned out to be the High Priest of the coven, Sarah merely laughs and tells her aunt that she has a story to tell her about why that message never reached her. Meanwhile, K9 tries to connect with the human holiday in his own way, teaching himself to sing "We Wish You a Merry Christmas".
