Starring Tom Baker, The Invisible Enemy is a four-part story featuring the screen debut of K9, the robot canine. When The Doctor answers a distress call from a shuttle crew who have been infected with an intelligent virus, he too becomes contaminated. The only solution is to create clones of The Doctor and his companion Leela (Louise Jameson) to enter his body and fight the virus.

K9 and Company sees the Doctor’s two companions, Sarah Jane Smith (Elisabeth Sladen) and K9 appearing together. Also known as A Girl's Best Friend, this was the first bona fide TV spin-off from Doctor Who.

In this one-off special, Sarah Jane pays a Christmas visit to her Aunt Lavania in the village of Moreton Harwood. But something is horribly wrong. Lavania is missing, leaving Sarah Jane in charge of her aunt’s ward Brendan. When Brendan is kidnapped by an evil cult of witches, Sarah Jane and K9 find themselves in the middle of a mystery.

Extras are as follows:
The Invisible Enemy:
Commentary - with actors Louise Jameson and John Leeson, visual effects designer Mat Irvine and co-writer Bob Baker.
Dreams and Fantasy - artistes and production crew recall the making of this story and even take the original K9 for walkies. With actors Louise Jameson and John Leeson, director Derrick Goodwin, co-writer Bob Baker, visual effects designers Tony Harding and Mat Irvine, K9 operator Nigel Brackley, journalist Gary Gillatt.