It features former series regulars Sarah Jane Smith, an investigative journalist played by Elisabeth Sladen, and K9, a robotic dog voiced by John Leeson.
The programme has its roots firmly in the desire of Doctor Who producer John Nathan-Turner to get Elisabeth Sladen back into the TARDIS.
Nathan-Turner's preferred plan for the transition from Baker to Davison was to have Sarah Jane be along for the ride from Logopolis to the second story of series 19.
The robot dog was very popular among children but was difficult to deal with technically and Nathan-Turner felt that it made the TARDIS crew almost overwhelmingly formidable.
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.
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.
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.
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.
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 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.
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.
Bill Fraser previously appeared with Tom Baker and K9 Mark II in the Doctor Who story Meglos in 1980 and Colin Jeavons played Damon in the Patrick Troughton serial The Underwater Menace in 1967.
"A Girl's Best Friend" featured electronic theme music, composed by Ian Levine and Fiachra Trench.
The proximate cause for this was a changeover in channel controllers at BBC One; Bill Cotton, who had approved the pilot, vacated his position soon thereafter.
A novelisation of "A Girl's Best Friend" was released under the title K-9 and Company, in October 1987, as the last of The Companions of Doctor Who series by Target Books.