Premise: Teenagers are stupid. They're always doing silly things that end up in someone dying, thus incurring the wrath of a killer fisherman. For some reason, it takes a year for said vengeance to occur, however, thus the titular reference to "last summer."
Potential spoilers ahead!

I Know What You Did Last Summer (1997)

Four North Carolina teenagers -- Julie (Jennifer Love Hewitt), Ray (Freddie Prinze), Helen (Sarah Michelle Gellar) and Barry (Ryan Phillippe) -- accidentally run over a man, then decide to dump his body in the ocean and never speak of it again. A year later, someone dressed as a fishermankills Helen and Barry. Julie does research into the man they ran over, David Egan, but realizes that it was actually Ben Willis, the father of David's dead girlfriend. The body the police found was David's, because Ben had killed him prior to being run over. Ben was actually alive when he was dumped into the water and survived to get revenge...one year later? Talk about procrastination. In the end, Ben ends up in the ocean, presumed dead. Again.More »

I Still Know What You Did Last Summer (1998)

One year after the first film, Julie is attending college in Boston, haunted by nightmares of the fisherman. Her roommate Karla (Brandy) wins a vacation in the Bahamas, and the two of them go with Karla's boyfriend Tyrell (Mekhi Phifer) and their new, un-suspicious friend Will (Matthew Settle). Somehow, even though it's early July, this is the end of the tourist season at the resort, so there are few staff members and no other guests. The fisherman shows up in North Carolina and attacks Ray, but he survives and hops a slow boat to the Bahamas, knowing that the vengeful fisherman will head down there. Of course, he does, this time with a little aid from his son...Will! Thanks to Ray and Julie, both father and son end up dead. Really.More »

I'll Always Know What You Did Last Summer (2006)

The scene in this tangential sequel shifts to Colorado. The fisherman has become an urban legend, a shadowy figure who supposedly stalks teenagers with dark secrets every 4th of July. This particular July 4th, a group of recent high school graduates pulls a prank that results in one of them dying. Panicked, the others vow to keep their involvement a secret and blame the death on someone dressed as the fisherman. A year later, someone dressed as the fisherman really does show up and proceeds to put the kids out of their guilt-ridden misery. How thoughtful of him. It turns out that the killer is an undead Ben Willis, who now has become a real-life urban legend, a vengeful ghost who punishes naughty teens, like an evil Santa with a hook.More »