Warning:Wikipedia contains spoilers
A group of teenagers go on a road trip in their car and accidentally run over a fisherman. They dispose of the body, thinking he is dead, only to receive a letter the next year stating, "I know what you did last summer". They are then -- some say predictably -- killed off one by one by a man who looks creepily like the one they mowed down.
Some accredit the popularity of the film to its characters and their subversive attitudes towards what is going on, arguing that plotwise it is fairly atypical of teen-slasher films. The prevailing view, however, is that it owed its success to the physical appeal of its young actors, all of whom were fresh faces at the time of its release.
Gellar and Hewitt have jokingly referred to this film as I Know What Your Breasts Did Last Summer, a reference to the fact that both wore push-up bras in order to give themselves enormously exaggerated breasts. Only recently have the women admitted that their breasts are much smaller than they appear to be in this film. One result of the film was the creation of a short-lived web-site devoted exclusively to Hewitt's breasts.
After the film's release, Prinze and Hewitt emerged as the two most popular young stars in the country. Prinze went on to do a string of romantic comedies aimed at the teenage market, but Hewitt's career foundered on a despised television biography of Audrey Hepburn and an ill-fated television series. Both are coming to be regarded as symbols of the superficial teen culture of the late 1990s.
Gellar and Prinze would later marry. Gellar has denied her interest in Prinze began when they met on the set of this film. Prinze was romantically involved with another woman at the time.
The film was followed by a sequel, I Still Know What You Did Last Summer.