I Know What You Did Last Summer is a classic 1990s slasher, but its ending twist revealing a human killer makes almost no sense at all. With the smash success of Wes Craven's Scream in 1996, the Hollywood floodgates were opened to a glut of studios playing follow the leader. For the next five or so years, slashers were back in vogue, especially if they took the same self-referential tone as Scream. Sadly, of this wave, very few copycats turned out to be all that memorable.
One of, if not the best of these was I Know What You Did Last Summer, ostensibly based on a book by Lois Duncan, but not to much of an actual degree. Oddly, I Know What You Did Last Summer failed to wink at the audience and make horror in-jokes like Scream, and was the better for it, as it made the film stand out from the pack. A great cast helped, full of 1990s notables like Jennifer Love Hewitt, Ryan Phillippe, and Freddy Prinze Jr., as well as Sarah Michelle Gellar right around when Buffy the Vampire Slayer made her a huge star.
I Know What You Did Last Summer also introduced a memorable new horror villain in Ben Willis (Muse Watson), a fisherman wearing a slicker and brandishing a hook murder weapon. While Ben is a great character, his reveal stretches the bounds of credibility.
I Know What You Did Last Summer's Killer Is Its Biggest Plot Hole
While I Know What You Did Last Summer's idea of a man being left for dead, only to survive and target his attackers later, certainly isn't a bad one, the way it's executed just doesn't logically work. The slicker-wearing killer, of course revealed to be local fisherman Ben Willis, repeatedly does things it would be almost or outright impossible for him to do. For one, how on Earth could he have put Max's dead body in the trunk of Julie's car, much less covered it with live crabs, then make it disappear when Julie brings the others to look just mere moments later? With a supernatural killer, this can be readily excused, but Ben Willis isn't superhuman, he's just a psychopath.
In another instance, Ben brutally murders Barry in a balcony above the stage where the annual town pageant is going on, and the only one who seems to notice it's happening is Helen. One could argue it was too loud to hear, but at the same time, how does Ben make Barry's body, and all but a tiny blood drip, disappear from the area in the time it takes Helen to make it upstairs? For that matter, how does he seemingly get ahead of Helen without her noticing when chasing her down an alley later? When it's zombie Jason Voorhees, teleportation can be shrugged off, but not with a living fisherman. One wonders if at one point the filmmakers considered making the killer supernatural, and just left those scenes in after the script got changed.