Hmmm. This one struggles to get going and has possibly the most un-menacing of antagonists I think I ever saw our time travellers have to face. The "Doctor" (Tom Baker) and "Leela" (Louise Jameson) respond to an emergency message from "Titan" and find when they get there that the crew have been infected by a nasty virus that makes a beeline for them too. Luckily there's the inventive "Prof. Marius" (Frederick Jaeger sans anti-matter this time) who has built himself a new best friend "K9" and who's a dab hand at cloning. The "Doctor" - infected himself, concludes that another him and then a bit off a cerebral rear-guard action is going to be required or else the "Swarm" is going to know everything he does - and that could well be dangerous! Minimalism goes scientific next as they both must avoid his passing thoughts and weave their way through the tunnels of his mind trying to track down the malevolent and cunning alien. Unfortunately, this isn't really the best of stories and as the creature manifests itself later in the series, it really does remind me of something that needs Marie Rose sauce. That said, it does introduce us all to everyone's favourite canine companion - complete with four levels of nose blaster, and successfully takes the Time Lord into a new generation of adventures. It's all watchable enough, but not a series that merits much remembering, sorry.