Two episodes from the Tom Baker era 'Doctor Who', devoted to the Doctor's favourite metallic pooch - K9. In 'The Invisible Enemy', a mysterious cloud is causing havoc in space, infecting all those who pass through it with an intelligent virus which could spread throughout the whole galaxy. The Doctor (Baker) and Leela (Louise Jameson) try to come to the rescue, but the TARDIS passes through the cloud itself, leaving the Doctor infected with the virus. He and Leela then try to find help at a nearby medical station, where they meet the eccentric Professor Marius and his pet robot K-9. In 'K9 and Company', the pilot from the 'Doctor Who' spin-off series, the Doctor's former companion, Sarah-Jane Smith (Elisabeth Sladen), arrives to take up residence with her Aunt Lavinia at the village of Moreton. She finds the house deserted apart from a crate marked for her attention, which turns out to contain a gift from the Time Lord: K9 Mark 3. The mechanical dog teams up with Sarah to investigate Aunt Lavinia's disappearance, and together they unearth evidence of witchcraft.