Psych - Episode 3.10 - Tuesday the 17th

Note: Spoilerfix has this listed as 3.09 - It is in fact 3.10

Jason, an old camp friend of Shawn and Gus comes to them saying he bought the camp they all had attended together and is reopening it soon. When he tells them that a counselor has gone missing, Shawn freaks out because the reason the camp was closed to begin with was a kid died there when they were younger. They agree to come out and try and help find the missing counselor, but when they get out there, things aren't looking like they thought they would. Jason seems like a totally different person, another counselor goes missing, they find bloody clothes in the dryer... Freaked out, Shawn calls Juliet out to help and when he finally figures out what is going on, what he reveals surprises almost everyone.

Source: SpoilerFix