I Know What You Did Last Summer is a 1997 horror film. It stars Jennifer Love Hewitt, Sarah Michelle Gellar, Ryan Phillippe, Freddie Prinze, Jr., and Anne Heche. The screenplay was written by Kevin Williamson, which was based on a popular novel by Lois Duncan. The film was followed with the sequels, I Still Know What You Did Last Summer and I'll Always Know What You Did Last Summer.

Kevin Williamson's screenplay was purchased before his screenplay for the movie Scream. It was only after Scream's success that producers rushed Williamson's screenplay for I Know What You Did Last Summer into production.

After teenager Helen Shivers (Sarah Michelle Gellar) wins the annual Croaker County Beauty Pageant, she and her boyfriend Barry William Cox (Ryan Phillipe) party on the beach with their respective best friends: Julie James (Jennifer Love Hewitt) and Ray Bronson (Freddie Prinze, Jr.). The four tell horror stories about a huge, hook-handed man who periodically kills young lovers. As they swerve home along a shoreline road, Barry spills his bottle of whiskey over Ray, who tries to clean the mess when a shape suddenly looms in the headlights. Before any of them know what has happened, they've skidded to a stop in the middle of the road. Although at first believing it was an animal, they realize that the dent on the car could not have been created from an animal. Julie finds a fisherman's boot on the side of the road and a few feet down the road, they see a man's body lying lifeless. They have been drinking, and if they turn to the police, all of their post-high school dreams will likely be shattered: no college football for Barry; no law school for Julie; no heading to New York for Helen or Ray. Barry, a bitter jock, convinces his friends to help him dispose of their victim's body, although the others would rather report the accident. The four swear to take this incident to their graves and never tell a soul about the incident.

A year later, after her first year at college, Julie returns home. She has been performing poorly in school due to the guilt she feels from the past incident. While spending time with her mother, she receives a mysterious letter which reads, "I KNOW WHAT YOU DID LAST SUMMER." Julie finds out that Helen has returned from New York because it "didn't work out." Julie believes that she never actually made it to New York. She now works at a department store, under the supervision of her elder sister Elsa (Bridgette Wilson). Ignoring Elsa's objections, Helen and Julie head off to find Barry. Judging by Barry's current condition, it is evident that he is the only one of the four who has felt little or no guilt from the incident. He immediately suspects Max of sending the letter. Julie remarks that they found the body of the man they killed last year, and he was identified as David Egan.

Max is murdered by a man wielding a hook, and Barry is run over by the same man. Julie arrives at the hospital and is greeted by Ray. They go to Barry's hospital room, where he and Helen await them. Doing some quick research, Helen and Julie discover that two years ago, David Egan accidentally killed his girlfriend, Susie, in the same location that they ran him over. They find the location of David’s sister, Missy Egan, and they decide to visit her for information. During the course of their conversation, Missy mentions Billy Blue, a young handsome man, who visited her, saying he was best friends with David Egan. Julie and Helen now suspect that David’s friend Billy Blue is trying to avenge his friend’s death.

Julie drives Helen home. The figure of Billy Blue steals into her room just before she enters. The next morning, Helen awakens to find herself wearing her old "Croaker Queen" crown. Most of her hair has been cut off and left on her pillow; the word "SOON" is written in lipstick on her mirror. Helen promptly calls Barry and Julie, both of whom she tells about the incident. While rushing over in her car, Julie hears a scratching noise in her trunk. She stops the car to investigate...and finds the murdered Max wearing Barry's stolen jacket. Crabs are crawling all over the corpse. Julie screams, shuts her trunk and flees for Helen's.

The next day, Helen is due to partcipate in the Croaker Queen Pageant, but is frightened that Billy Blue will attack her. Before the pageant, Barry holds Helen tightly and promises that everything will be okay. During the pageant, however, Billy Blue attacks Barry from behind on the balcony. Helen sees Billy dragging Barry off the balcony and out of sight. She begins to shout and cry, but it's of no use; she is held back by the crowd, thwarting her attempts to go and rescue Barry. The distraught Helen is driven home by a police officer, who unfortunately stops to see if a stalled car needs help. The other driver turns out to be Billy Blue, who kills the cop. Helen screams, jumps out of the car and flees for her life to Shivers Department Store. Billy is already there, having "hooked" Elsa when she went to lock the storage room-door. Helen just has time to discover Elsa's corpse before she is discovered by Billy. Brandishing his signature murder-weapon, Billy stalks Helen through the empty store and its storage rooms...finally cornering and butchering her. Loud music from the parade bands drowns out Helen's death screams.

Meanwhile, Julie at last figures out David Egan's true identity. After meeting with Ray, she solves the mystery. They didn't run over David Egan one year ago, but someone else. Because the face was disfigured and bloody, they were unable to truly identify who the person was; Julie believes that Ben Willis, Susie's father, tried to avenge his daughter's death by killing her boyfriend/murderer, David Egan. Missy, however, believed that David committed suicide due to guilt and depression. David was actually murdered by Ben Willis and was set up to look like a suicide. His body was the one found by the police in the docks. Ben Willis was the man that Julie and her friends ran over; however, he lived and returned to kill them. The fisherman is Ben Willis; this is revealed to Julie when he pulls out the large fishing-hook from his jacket. The older man is also wearing dirty Wellington boots, the same boots Julie found at the incident a year ago. She finds the corpses of Barry and Helen in the ice meant for freezing fish. The chase ends with Ben getting his hand chopped off, and then thrown overboard. Although later, nobody can find his body, only his hand and the fish hook. The sheriff says not to worry and that they “usually” do find the body.

A year later, Julie is back at college. (Ben Willis’s body remains missing.) She prepares to take a shower after finishing a telephone conversation with Ray. A dorm-mate passes by and tells Julie that she has mail: a letter which looks exactly like the one Julie received last year, the letter which started her entire nightmarish ordeal. To Julie's relief, it is only an invitation to a pool party. Returning to her shower room, however, she finds it completely filled with steam. She also finds the words "I STILL KNOW" written on the glass shower-door. Julie is shocked. Then Ben Willis, still wearing his slicker, crashes through the mirror.

Differences Between the Movie and the Novel:
In the book the victim's name is David Gregg, not David Egan.
Barry was driving in the book.
In the book it was a hit and run accident and never a cover up.
Elsa and Helen's last name is changed from "Rivers," in the novel, to "Shivers" in the movie.
The novel is set in New Mexico, while the movie is set in North Carolina.
In the movie, Elsa is quite pretty even with glasses (as might be expected of the real-life Miss Teen USA from 1990). In the novel, Elsa is homely and overweight. She is also bitterly jealous of Helen, repeatedly pointing out that if it wasn't for Helen's "petty" status as "Channel Five's Golden Girl," then Helen (supposedly) would neither be nor have anything worth talking about.
In the novel, Julie is a year younger than Barry and Ray, and is at the end of her senior year in high school when the book starts. She has just been accepted by the prestigious Smith College. In the movie, Julie graduates the same year as Barry and Ray, and is struggling with low grades at her college.
Barry and the Shivers/Rivers sisters do not die in the novel.
Helen is thrown a couple of stories from the bathroom window of her condominium; although now crippled, she survives to report the incident. Meanwhile, Elsa simply disappears from the story.
Barry is shot in the abdomen and has to be fitted for a colostomy bag, thereby ending his sports career before it begins; ultimately, he dissolves "the Pact" by agreeing to report what he and his friends did last summer, when it becomes only too clear that all four of their lives are being jeopardized in every sense.
In the novel, Helen works as a weather girl for the local news station; she is successful enough that she can afford to live at "The Four Seasons," a swanky apartment complex. In the movie, Helen lives at home after failing to make it as an actress in New York, and she works for her father's department store.
Billy Blue, Ben Willis and Max are never mentioned in the novel.
In the novel, Helen and Elsa come from a large family. In the movie, it appears to be just the two sisters and their dad, although their mother was mentioned.
Instead of chopping off the Villain's hand or throwing him into the ocean, Ray--in the novel--knocks the Villain's head in with a flashlight (incapacitating but not killing him).
In the novel, the teens' victim was a young boy on a bike, whose elder brother comes seeking retribution.
The novel's Villain is not a fisherman and never uses a hook. Moreover, in the novel, he seeks to murder only Julie. This is to punish Ray, since--as Ray himself tells Julie--"He knew that the worst thing for me would be to stay alive in a world without you."
In the book, the Shivers/Rivers sisters have very little money; in the movie, they have plenty of money.
In the novel, Ray Bronson's parents are mentioned multiple times; in the movie, his family is supposedly dead.
In the book, Ray's family owns a sporting-goods store, and is somewhat wealthy; in the movie, Ray is poor.
In the novel, Barry Cox is in the hospital for well over a week; in the movie, he is out of the hospital within a day or so, with a broken arm.
Max could be a replacement for Bud, aka Collie.
In the novel, Barry is cheating on Helen; in the movie, they split up for a while and then get back together.
It should be noted that the novel and movie are almost entirely different.

Opening to $15.8 million in 2,524 theatres, the slasher was declared a hit from the beginning. Impressively, the movie stayed in the #1 position again the following week, throughout October 1997. The end result was a total of $72.5 million and making its mark as one of the few popular horror films released in the 90's.

Critics, however, were mixed in their reviews. Critic Roger Ebert wrote in his review, "The best shot in this film is the first one. Not a good sign."

After the film's release, Prinze and Hewitt emerged as two of the most popular young stars in the country. Prinze went on to do a string of romantic comedies aimed at the teenage market. However, Hewitt's career initially floundered in the late 1990s after a poorly reviewed television biography of Audrey Hepburn and an ill-fated 1999 television series. However, at the turn of the millennium, she rose to fame as an actress, working in movies alongside Jackie Chan, and most recently, receiving a starring role in the hit CBS Television Show Ghost Whisperer.

Meanwhile, Gellar's was already one of the most popular female actresses in America, and her role in the movie, alongside starring on her hit TV show Buffy the Vampire Slayer, began a long career of horror films and television shows for the actress. She furthered her horror career by appearing later that year in Scream 2. She would later go on to star in 2004's The Grudge and also in 2006's sequel The Grudge 2 and the horror thriller The Return. Phillipe co-starred with Gellar in Cruel Intentions.

Gellar and Prinze would later marry. Gellar 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. Prinze and Gellar later costarred as Fred and Daphne on the movie Scooby Doo and its sequel.