RKMBs Forums .::Robegories::. Media The Official Doctor Who thread

This month's issue of Rue Morgue has an article on Doctor Who. It wasn't that long but it was a decent primer for new fans. I used to casually watch the Tom Baker episodes. There was one where there was what I think was a giant space caterpiller and it infected people who would then become caterpillar-like. I only saw up to the third part. Does anyone remember this - what it was called? I'd love to get it on DVD - the final scene of the half-caterpillar/half human yelling at the Doctor gave me nightmares for days!