This was the very first Doctor who spin off. It was transmitted as a one off episode on the 28th of December 1981. It starred two of the most popular Doctor Who companions, Elisabeth Sladen as Sarah Jane Smith and of course K9. the episode was called ' A girls best friend' it was meant to go into a series but never took off.
I remember the episode when Sarah had K9 given to her in a box as a gift from the 4th Doctor, and the theme music played a little as K9 mentioned the Doctor, and she said, 'oh Doctor, you didn't forget'. I also remember the end where K9 sings 'we wish you a merry Christmas.