South Park: Season 5, Episode 14

5 - 14: Butters' Very Own Episode

Butters wants his parents to have a nice wedding anniversary, which is coming up in a few days. His mom asks him to spy on his father to find out what he's getting her for an anniversary gift, but Butters discovers that his dad goes to some very disturbing places. His mom is shocked, and decides to take drastic measures to make things right again.