Okay, a little bit of a cheat since it wasn't broadcast as part of a play 'strand', such as Play for Today or Armchair Theatre, but here, for your delectation, is K9 and Company, the first ever Doctor Who spin-off and arguably the best of them all.
First broadcast on 28 December 1981, this pilot for a TV show that would never materialise saw Elisabeth Sladen return to her most famous role, Doctor Who companion Sarah Jane Smith, now safely returned to Earth and living a life for herself once again as a journalist. But when she goes to visit her Aunt Lavinia in the country, she finds her aunt is missing – but Lavinia has left behind a present from her friend the Doctor… Can you guess from the title what it is?
The show was an attempt by then Doctor Who producer John Nathan Turner to solve two 'problems' – the first was that he wanted to get Lis Sladen back into Doctor Who, but she was unwilling to be just a companion again; the second was what to do about Doctor Who companion K9, who was logistically problematic. As a result, JNT wanted to write out the robot dog but he/it was incredibly popular. K9 and Company, in which Lis Sladen would be the Doctor figure and K9 a helper, was his solution.
And, actually, it's pretty good. Written by former BBC producer Terence Dudley (Doomwatch, Survivors), beyond K9, it doesn't touch on science fiction at all, instead being a spooky Christmas mystery involving what appears to be the supernatural. Sarah Jane gets some character development and family background – and gets to beat up a baddie (in Dudley's novelisation of the story, it's suggested she's now a karate black belt). And it's an enjoyable 50 minutes or so.
The show got above average ratings – 8.4m, which was more than the average episode of Doctor Who was getting at the time. But unfortunately it was a victim of BBC politics: commissioned by Bill Cotton, it was disliked by his replacement as BBC1 controller, Alan Hart, who chose not to commission a series.
But in many ways, the show lived on and was not forgotten. In the Doctor Who story The Five Doctors, both Sarah Jane and K9, now living together in a suburban house, make an appearance. School Reunion reintroduced Sarah Jane to the new series of Doctor Who and she had K9 with her – by now, broken down and in need of repair by the Doctor. And, of course, Sarah Jane got another spin-off series, The Sarah Jane Adventures, in which a new and improved K9 made at first occasional appearances before becoming a regular in the third series.
But here, for your delectation, is the full thing, which is also available from Amazon: buy it if you like it! One word of advice: even if your eyes can withstand the title sequence, your ears probably won't be able to take the theme tune, so probably best to mute it.