I watched this one yesterday, and quite liked it. As a whole, this story is fast-paced, and I'm surprised Doctor Who's only ever done the 'shrink down and enter an individual' twice on-screen. The Nucleus's presence looks great on camera, even if the creature itself looks a bit silly. I did enjoy the feeling that you'd never know who was infected, but it could've been done a bit better. Overall, I enjoyed this story, and I'd love to see more from the Nucleus. Perhaps I'll have to write one.