This was the pilot for a never-realised spin-off of Doctor Who (1963). Elisabeth Sladen's character of investigative journalist Sarah-Jane Smith had been the Doctor's travelling companion from 1973 to 1976, whilst K-9 had been introduced in 1977. This version of K-9 was mark III. The first had stayed with Leela on Gallifrey, the second stayed with Romana in E-Space, and the Doctor made this version as a gift to Sarah-Jane Smith.
This was watched by 8.4 million viewers on its original transmission.
Elisabeth Sladen revealed on the DVD commentary that she found John Black's direction flat and lifeless.
The BBC decided not to go ahead with the proposed spin-off and not commission the first season. Sarah Jane Smith would finally get her own spin-off series The Sarah Jane Adventures (2007) 25 years later, where she was still K-9's owner.
K-9 later made a brief cameo appearance in a scene with Sarah Jane Smith in The Five Doctors (1983), endorsing the canonicity of the edition.