- Lennon Grant returns home after her first year at college and discovers that someone knows the terrible secret she and her OG crew of friends swore to keep last summer.
- Lennon returns home to Hawaii following her freshman year at the University of Michigan. She's greeted by her dad, Bruce, who runs a restaurant and hotel, but brushes off his cheerful greetings. She also ignores phone calls from her friends. Going to her room, she's interrupted by Courtney, who works as a cleaning lady for her father. Lennon asks after Courtney's daughter, Riley, one of her friends, and dismisses any need to clean her room. When Lennon goes to unpack she finds the phrase "I know what you did last summer" written on the mirror inside her closet. Seeing blood on some of the hanging clothes she looks behind them and screams.
One year earlier, Lennon is at a massive party thrown by her best friend Margot. Lennon is supposed to be going away to college tomorrow. As Lennon and Margot dance, they see her father arrive with Lennon's identical twin sister Allison, who does not yet have a driver's license. Lennon grabs Allison and takes her to a bathroom to do her makeup, something Allison resists, since it will make her look more like Lennon. On the way, they bump into Riley, who is with Dylan, a friend who Allison has had a crush on for years. Lennon gives Allison a makeover and crudely encourages her to try and seduce Dylan. Allison rejects her sister's advice. Allison berates Lennon for using sex to get what she wants, while Lennon retorts that it's better than "having no life" like Allison. Allison says that she doesn't want to be like Lennon because Lennon is "the worst person I know" and storms out.
Allison sits on the shoreline and watches the party from afar. She's joined by Johnny, one of her and Lennon's few mutual friends. The two commiserate, and Johnny encourages her to talk to Dylan. Meanwhile, Lennon finds Dylan and takes him to one of the bedrooms where she offers him drugs. Allison returns to the party, finds Dylan, and the two begin talking. Lennon barges in and begins talking flirtatiously with a tongue tied Dylan. Allison, realizing her sister had sex with Dylan, storms off.
Riley, who has been distributing drugs to people at the party, sees a police officer arrive. She, Margot, Dylan, and Johnny decide to leave. When they go outside, they find Lennon sitting in her car. They all pile in and tell her to drive. As Lennon backs out, she hits a mailbox and then drives erratically down the street. Dylan asks Lennon if Allison is okay, to which she replies "No." As the other friends, chatter, Dylan repeatedly tries to talk to Lennon about what they did, but she rebuffs him. As they drive along an isolated coastal road, the car hits something. As they stop to look, Dylan insists that it was probably one of the wild goats which roam the area. Lennon says that she doesn't think it was a goat. Riley walks ahead and finds what they struck. As Lennon comes up to look, the other friends try to keep her away. It's her sister, Allison. Lennon collapses and sits catatonic on the side of the road. They see a car approaching. It's their friend Dale. He stops and talks to them, but doesn't seem to suspect anything is wrong. The friends debate what to do. Johnny wants to call the police and take responsibility for what they did. Dylan is amenable to that too. Riley and Margot are adamant that they not call the cops. Riley insists that they'll all be arrested for vehicular manslaughter and for the copious amounts of narcotics in the car and that all their lives will be ruined. The girls suggest that they hide the body and pretend that Allison, who Riley describes as having no friends and no prospects in life, simply ran away. They see the lights of another car approaching, but it turns off the road before it gets to them. Margot says that Lennon should decide what they do, since it's her sister and she, as the driver of the vehicle, faces the most legal jeopardy.
Lennon decides to go along with the other girls and hide Allison's body. Following a suggestion from Dylan, they take her to a seaside cave. Years ago, Lennon and Allison's mother had died in that same cave as part of a mass suicide committed by a cult which she was a member of. Remembering that the tides had carried many of the bodies away, Dylan suggests that the same thing will happen to Allison, making it harder for anyone to find her. The friends drive to the cave site. Along the way they hear a noise from the trunk. Thinking Allison might be alive, they check, but find that it's just the ringtone on her phone. They take Allison's body to the cave, which is covered in carved names of those who died there. Johnny insists they say a prayer for Allison and as they do so, Riley thinks she hears someone behind them, but it's just rats. As the tide comes in, the friends leave the cave.
In a flashback to the party, Allison confronts Lennon about having sex with Dylan. Allison hates her sister for this betrayal and accuses her of trying to take everything from her. Lennon replies that Allison should live a life and not be so sad and depressed. Allison replies that she's sad because her mother died, something that Lennon has moved on from. Allison insists that Lennon was always their dad's favorite, while the only one who loved Allison was their mother. Lennon tells Allison that their mother didn't love her, but she does and that Allison is now free to get on with her life. An angry Allison takes off her necklace, which she wears in memory of their mother, and throws it at Lennon, telling her "now you have everything". Allison stalks off. A tearful Lennon puts on Allison's trademark necklace. Finding her own jacket pinned beneath two people who are making out, Allison grabs Lennon's jacket and gets into her car, intending to drive home. Just then the friends pile in and, mistaking Allison for Lennon, tell her to drive. It was Allison who was driving the car. The person they hit, the one everyone thought was Allison in part because she was wearing her necklace, was Lennon.
In the present, "Lennon" finds a severed goat's head hanging in her closet.
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