The film opens at a boarding school during the big party that signals the approach of summer break. While most students spill outside to hear the headmaster’s speech, a young girl named Rebecka is seen in the showers, writing a letter, before she inexplicably climbs to the tower and ends her life. A year passes, and the narrative follows Sara, who witnessed the suicide, as she returns for her final year. Two new students arrive—Felix, a sharp-minded math genius, and Leo, the son of a diplomat—adding fresh dynamics to the tight-knit group.
Sara and her friend Therese begin spinning the local legend of Strandvaskaren, the Drowning Ghost. The tale says that a farmer slaughtered three male students a century earlier and then drowned himself in the nearby lake; his spirit is said to return every year on the anniversary of his death to haunt the school. To mark the day, students hold a party in the barn where the farmer once lived, perpetuating a chilling tradition.
As the mystery deepens, Sara discovers a dark motive behind the century-old killings: the farmer murdered the students to avenge his daughter’s rape and murder, a crime that authorities reportedly shielded because the rapists were from the upper class. At the party, the mood darkens when Måns and his friend Ynge attempt a prank on Sara, but a drunken Ynge makes a crude move toward her. Sara pushes him away, and Måns yells at Ynge for taking things too far. The shock continues when Sara later finds Måns’s current girlfriend dead in the hallway, and Ynge is killed—this time by a fork driven through his head, after he drinks milk.
A masked figure then bursts in and drives an axe into Måns’s head as Sara scrambles to safety. She locks Leo in a storeroom and flags down Therese and Felix to help. In the midst of the chaos, Sara finds an 8mm film in a box once owned by Rebecka and learns that Rebecka had a brother who bears a striking resemblance to [Felix]. Felix confirms that he and Rebecka were siblings, separated years earlier when their father, mentally unstable, was committed. He confesses a vow of vengeance and, donning the Drowning Ghost guise, begins to target the students who tormented Rebecka. He pleads with Sara not to expose him, certain that she would.
Sara knocks him out and escapes with Therese, who then falls into a well and breaks her leg. In the morning, Felix awakens to find Sara and Therese still trapped in fear and unable to seek help; they crawl down a pipe toward the lake. Felix catches Therese and kills her off screen. Sara finally reaches the lake and slips into a boat, keeping her eyes on the pipe as she waits for Felix to emerge. She dozes, and when she wakes, he is nowhere to be found. Felix surfaces and tries to climb into the boat from underwater, but Sara starts the engine and severs his grip by cutting his hand. He makes a last, brutal lunge, but she defeats him by bashing the anchor into his back, sending him to the lake’s bottom.
The police arrive to rescue Sara, and she is joyfully reunited with Leo on the beach. The investigators obtain the farmer instead of Felix, suggesting the legend may have more truth than fear. Sara and Leo eventually graduate in Stockholm, and eight months later, during the graduation ceremony with countless families watching, a red-painted message appears among the crowd reading My beloved sister Rebecka. The film closes with Sara realizing that Felix is still alive among the spectators, even if she cannot see him in the crowd as she and her classmates step into their futures.
