Lennon watched over a cliff with lots of waves. Her voiceover states that she doesn't know who her friends are or their true selves. It's an ominous foretelling about darkness and facing evil, and the secrets they're keeping.

They live in the town of Wai Huna, Hawaii. Lennon is back from the University of Michigan.

Lennon receives a text with a clown emoji; she nearly runs over a dog. She has an awkward reunion with her dad, Bruce. Lennon hasn't been back home in a year.

Lennon gets scared by Courtney. After Courtney leaves, she opens the closet door to find a message on her mirror: "I Know What You Did Last Summer." She notices blood dripping on the floor and clothes; when she pulls the clothing rack back, she screams.

The episode shifts to the previous summer. There's a big party going on for the teens; Riley pulls Dylan away to conduct business. Riley did a drug deal.

Allison is unsure if she wants to join the party. Meanwhile, Margot and Lennon are dancing on stage; Margot is sad that Lennon is leaving for school.

Allison is upset Lennon left for the party without her. Allison is shy when she bumps into Dylan; she heads into a bathroom. Riley gives Lennon drugs.

Johnny and Riley take photos at the party. Margot's mother pulls them over to meet her friends.

Lennon encourages Allison to sleep with Dylan.

Johnny, Margot, and Margot's parents film a TikTok. They discuss that Johnny and Margot are going to USC; Allison is staying in Hawaii.

Allison doesn't know if she wants to sleep with Dylan. Allison and Lennon get into an argument about their lives; Allison reveals that Lennon sleeps with everyone, including her AP Bio teacher to pass her course.

Johnny reveals he hooked up with Gary during Junior year when he was with Margot.

Lennon takes the drug and spots Dylan. She convinces him to take the drug with her.

Johnny meets up with Allison at the end of the cliffs. Johnny is keeping a secret from Margot: he got accepted into Julliard and he's going to that school in the fall. Allison and Johnny are smoking.

Allison and Dylan awkwardly flirt. She asks him to go talk somewhere and do a shot together. Lennon interrupts their chat; she takes the shot from Allison and gives Dylan a knowing look. It's revealed that Lennon slept with Dylan at the party.

A police chief arrives, which causes Riley, Margot, Dylan, and Johnny to panic. They get into the car with Lennon and drive away; the others in the car are drinking or getting high.

The car hits someone: it's revealed to be Allison.

Dale, a friend of the group, drives by and asks them if they need help. Before Dale gets out of the car, Margot starts throwing up and Dale drives away.

Riley, Dylan, Margot, and Johnny debate what they should do next. Johnny and Dylan want to come clean, but Margot and Riley want to keep it a secret. Dylan thinks they should hide the body in the caves.

A flashback shows Lennon and Allison arguing in the closet at the party. Allison says she knows that Lennon cuts herself.

The group brings the body to the caves, but Clara is watching from a distance. The group finds the names of victims carved into the walls of the cave; the names also include Allison and Lennon's mother. The group says a prayer for Allison.

As the tide rolls in, the group leaves Allison's dead body in the cave. Riley hears a sound coming from the distance.

In the flashback, Lennon tells Allison that their mother didn't love her. Allison throws her mother's necklace at her. Lennon puts on the necklace. Allison takes Lennon's jacket and gets in the car.

It's revealed that Allison was behind the wheel of the car and is pretending to be Lennon; Lennon was the twin who died and got hit by the car.

In the present timeline, Allison finds a severed ram's head hanging in her closet.