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...