Summary
Highlights
Ava returns to her hometown for an engagement party, where she reunites with her ex, Milo, and high school friends Danica, Teddy, and Stevie. After the party, they head to watch fireworks. A drunk Teddy accidentally causes a car to crash, killing the driver, Sam Cooper. Teddy convinces the group to cover up the accident, fearing the consequences due to his influential father. Ava reluctantly agrees.
A year later, Ava returns to Southport again and meets podcaster Tyler, who is investigating a 1997 massacre. At Danica's bridal shower, she receives a gift card with the message, 'I know what you did last summer.' The friends suspect Teddy, but he denies involvement. Milo speculates it could be related to Sam Cooper.
Danica's fiancé, Wyatt, is brutally murdered by a mysterious figure in a raincoat wielding a hook. The killer leaves a message: 'You can't erase the past.' The group suspects Tyler's podcast, which details a 1997 massacre where a fisherman killed teenagers trying to cover up a murder, beginning with a similar anonymous note. Teddy reveals his father, Grant, a powerful developer, erased case files from the 1997 incident.
Ava joins Tyler to investigate the 1997 massacre at Bayside House, one of the original crime scenes. Tyler mentions that the 1997 killings began after someone received the note, 'I know what you did last summer.' Ava is attacked by the fisherman, but Tyler manages to escape, only to be killed shortly after. Ava escapes the house and reunites with Stevie.
Ava contacts Julie, a survivor of the 1997 massacre, who advises her to find the killer. Stevie leads the group to Ray, another 1997 survivor and Julie's ex-husband. Ray suggests investigating the owner of the truck Sam was driving. They discover it belonged to Pastor Judah Gillespie, who was close to Sam. Later, Milo is murdered by the fisherman while at Teddy's house.
Ava and Danica investigate Pastor Judah, realizing he is hiding something about Sam. They try to inform the police but are held by Grant. Teddy is attacked and killed by the fisherman, and Grant is also murdered when he tries to help his son. The bodies of Teddy and Grant are displayed publicly.
Milo's body is found, and the girls, along with Ray, plan to escape by boat. Police find Pastor Judah murdered, and a shocking photo reveals Stevie with Sam and Pastor Judah, implying a deeper connection. On the boat, Stevie is revealed as the fisherman. She explains her grudge against the group for abandoning her and her grief over Sam's death, who was like a brother to her. Stevie attacks Danica, throwing her into the water, and is then shot by Ray.
Ava calls Julie, revealing Stevie was the killer, but Julie finds it suspicious. Ava realizes Ray is also involved after seeing a wound on his arm, which she inflicted on the fisherman. Ray confesses, revealing that Stevie is still alive and that he is the true mastermind, seeking revenge for the 1997 massacre and his perceived betrayal by Southport. He attacks Ava.
Julie arrives and confronts Ray, who explains his motive, but Ava shoots him with a harpoon, killing him. In a shocking twist, Danica is found alive on shore. The film ends with Ava and Danica joking about Stevie, revealing she is still alive and they might have to kill her before she hunts them down.