Now available on DVD as part of the K9 box set, this is the story where The Doctor and Leela first encounter the robot dog; a substitute pet for Professor Marius, a scientist working at the Bi-Al Foundation, an asteroid circling Titan, one of the moons of Saturn. The titular enemy is the leader of 'the swarm', a space virus that infects the crew of a spaceship destined for Titan and which plans to use the moon as its new breeding ground.
The story is notorious for its 'giant prawn'; an enemy that probably ought to have remained invisible as the SFX available to the production team certainly weren't up to bringing it to life (as an avid reader of the Target Doctor Who novelisations I was misled by the cover's superb depiction!) This aside, there are some good performances and some memorable moments; apart from K9 there is the redoubtable Michael Sheard - Mr Bronson from Grange Hill and a Doctor Who staple in the 70s - and a great sequence where The Doctor and Leela are cloned and injected into the Doctor's bloodstream to try and defeat the virus that has infected The Time Lord.
Overall, a great idea that is let down by its SFX but which is still a fun and engaging adventure in classic late 70s stylee.