Need your help to find out one single chapter

Hello,
When I was travelling outside (I think it was on UK) I saw a part of one single chapter and I want your help to know which was it (season/number) to find it easily. I just remember the basic plot: Some people is turning into pod-people, moving like zombies and singing a very stupid song. If somebody knows what I am talking about please help! (otherwise I mistook the series name, sorry).
Thanks!