It was one of the Pirates Who Don't Do Anything episodes where the crew and several others were stranded on a desert island. They ran out of food and had to decide who to eat based on who wasn't living by Jesus' teachings. The weirdest part was that it kept switching between parodying Gilligan's Island, Lord of the Flies, and Lost. The plot was really hard to follow up until the cannibalism musical number, then it kinda fit together.

Honestly I'm surprised they even released it as it was since, as far as I remember, it's the only episode that showed any actual violence, even in a toned down PG kind of way.