This story seems to be mainly be remembered as the story that introduces K-9 but apart from that I've never heard much good said about it. I watched it the other night and found it immensley enjoyable! It's really far out and imaginative, the sheer scale of the story is hugely ambitious and it's a credit to the production team that they were able to realilze this story at all. The notion of making a clone of someone, shrinking them and then injecting them into the same person's head is bonkers but brilliant! The sets for the inside of the Doctor's brain are very good and rather psychedelic. The concept of the Virus is chilling, unfortunately the realisation is unintentionally hilarious (a giant prawn!) although the voice is pretty good. The space model effects are excellent, easily on a par with the mush more expensive series Space 1999. The Invisible Enemy really is one of the best examples of pulpy science fiction space opera presented by Doctor Who, Tom is on great form as is Louise Jameson, K9 comes across well in his debut, the possessed human's are quite chilling and the whole story is exciting, entertaining, funny and imaginative. A very underrated story.