The synopsis: A little boy named Jason Voorhees drowns while swimming at Camp Crystal Lake in the '50s because the camp counsellors are distracted, and his body is never found. After a few of the counsellors are mysteriously murdered, the camp shuts down for over two decades. Upon its reopening in the '70s, the killer returns to take down the fresh crop of counsellors.
The twist: Jason's mother, driven crazy by the death of her son, went on a killing spree out of revenge (the original counsellors were too busy getting it on to notice Jason drowning). Luckily the final survivor of the massacre, camp counsellor Alice, kills Mrs. Voorhees. Not so luckily, she decides the best way to decompress after all that murder is to paddle a canoe out on the lake. All of a sudden, Jason's shrivelled, gross, Gollum-like body leaps from the depths and drags her overboard. She gets out with the help of some cops, and Jason is never seen again (well, until the 300 or so sequels that came out afterwards).
The amount of damage it'll do to your psyche: 🔪🔪🔪🔪