Although K-9 and Company was conceived as a spinoff of Doctor Who, it wasn't picked up, airing as a single special, also called "A Girl's Best Friend". Nonetheless, K-9's subsequent appearances with Sarah Jane in The Five Doctors and "School Reunion" confirm that the special is part of the Whoniverse Canon, and many elements of it define the character of Sarah Jane Smith after her departure in "The Hand of Fear". More than 25 years later, a second, more successful spinoff for Sarah Jane was made in The Sarah Jane Adventures, with K-9 making occasional appearances. Subsequently, K-9 also got his own show in Australia. (However, some of these stories feature different K-9 robots - the K-9 of the solo series is Leela's K-9, last seen on TV in "The Invasion Of Time". And the K-9 solo series was made with the rights to the character but not to the Doctor or the larger Whoniverse, making its canon status ambiguous.)
Sarah Jane Smith spends the Christmas holiday at her Aunt Lavinia's house in Moreton Harwood to watch her ward, Brendan Richards. In the house, Sarah Jane spots a large crate that was left in the attic for a few years. Inside is K-9 Mark III, a present from the Doctornote
There's also a cult going around doing nasty cult stuff, like kidnapping Brendan. Sarah Jane and K-9 investigate and find out that almost everyone in town is a part of this cult, called the Cult of Hecate. At night, the Cult of Hecate prepare Brendan as a sacrifice for the Winter Solstice, but Sarah Jane and K-9 save him just in the nick of time.
That's pretty much it really. Can you imagine what a full series would be like?
K-9 and Company provides examples of the following tropes:
- Accent Upon The Wrong Syllable: Brendan pronounces "motherboard" as two words.
- Annoying Laugh: Brendan's laugh is described by Sarah Jane as "honking".
- Aw, Look! They Really Do Love Each Other: The Doctor and Sarah Jane parted a little coldly, given her desire to go home. His message to her from K-9? "Give Sarah Jane Smith my fondest love. Tell her I shall remember her always."
- Black Cloak: The members of the Cult of Hecate wear them.
- Christmas Episode: The episode takes place in the days leading up to Christmas 1981. In the final scene, K-9 sings "We Wish You a Merry Christmas".
- Clarke's Third Law: George believes K9 to be Hecate's "fire-belching" familiar.
- Continuity Snarl: K-9 mentions being dropped off in 1978 (two years after "The Hand of Fear" aired), even though the UNIT stories supposedly took place a few years in the future and Sarah herself previously referred to her own time as being 1980 in an episode from 1975. Mind you, if the Doctor dropped Sarah off at the wrong place, he could easily have been a few years early too.
- Cult: And almost every townsperson (at least the ones we see) is in it.
- Curb-Stomp Battle: K-9, a Robot Dog from the year 5000 and armed with a laser, versus a human-sacrificing cult armed with chants. The results should be obvious.
- Distressed Dude: Brendan is kidnapped by the Cult of Hecate.
- Fright Deathtrap: Used on Sergeant Wilson.
- Human Sacrifice: Brendan is almost sacrificed by the Cult of Hecate but Sarah Jane and K-9 are able to rescue him in the nick of time.
- Innocuously Important Episode: It's not much by itself, but this is the episode in which Sarah Jane gets a K-9 from the Doctor (K-9 was introduced on Doctor Who during the tenure of Sarah Jane's successor Leela.). And much of the information here about Sarah's childhood and her aunt Lavinia would actually be remembered and developed further in The Sarah Jane Adventures years later - not least that Sarah Jane shares her aunt Lavinia's habit of caring for "strays" and orphans and that everything that keeps Sarah Jane going in that series, the house, money, etc, comes from Lavinianote .
- Just in Time: Sarah Jane and K-9 make it in time to stop Brendan's sacrifice. K-9 goes full salvo.
- Kid Sidekick: Brendan is one to Sarah Jane.
- Mundanger: What is the first threat Sarah Jane Smith and K-9 tackle together? Religious fanaticism.
- Police Are Useless: Fobbing off Sarah Jane because Sergeant Wilson is a coven member himself.
- Race Against the Clock: Sarah Jane and K-9 must save Brendan before he is sacrificed by the Cult of Hecate.
- Red Herring: It is strongly implied that Howard and Juno Baker are members of the Cult of Hecate but it turns out that they are the only named residents of Moreton Harwood (bar Sarah Jane's aunt Lavinia) who don't belong to it.
- Robot Dog: Brendan thinks that K-9, a talking robot dog, is the best thing that he has ever seen. Given that this is the third K9 model the Doctor has owned in some capacity, evidently he does too!
- Scare Chord: Whenever someone in the opening scene's ritual appears.
- Secret Circle of Secrets: The Cult of Hecate form one as they prepare to sacrifice Brendan.
- Spotlight-Stealing Title: A confusing case. Based on the title, one would assume that K-9 is the main character and end up wondering why Elisabeth Sladen is credited first in the title sequence instead of John Leeson. This really is Sarah Jane's show, but K-9 gets his name in the title instead. (BBC executives insisted K-9's name was in the title of the show - John Nathan-Turner wanted to call it "A Girl's Best Friend", which became the title of the first and only episode instead.)
- Title Theme Tune: This and Class are the only Whoniverse shows with vocal lyrics in their theme songs. In the case of this one, the lyrics are just John Leeson saying "K-9!" ad infinitum.
- Took a Level in Badass: Looks like Sarah Jane knows martial arts now.
- Town with a Dark Secret: Moreton Harwood is secretly home to the Cult of Hecate, a coven of witches who practice human sacrifice.
- Tuckerization: One couple is named Baker.
- When the Clock Strikes Twelve: Midnight on the Solstice is Sarah Jane and K9's deadline for saving Brendan.
- Who's on First?:Brendan: Who is the Doctor?
- You Have to Believe Me!: Sarah Jane cannot convince anyone about the coven.