The film opens with Gong Ju (Chun Woo Hee) courageously standing before a crowd of disapproving adults, asserting that she has done nothing wrong. The narrative swiftly transitions to her compassionate teacher, Lee Nan Do (Jo Dae Hee), who lends a helping hand as she prepares to transfer to a new school far from home. During their lunch, he reassures her that the trial's outcome will not reflect her guilt and cautions her against answering phone calls during this tumultuous time. Lee does his utmost to make the transition easier for Gong Ju, even introducing her to a local convenience store run by his mother, Mrs. Cho. Initially resistant, she eventually consents to let Gong Ju stay at her house.
In a poignant flashback, Gong Ju is seen working as a cashier when her friend Dong Yoon (Choi Yong Jun), son of the store's owner, enters looking battered. He hastily grabs food and exits to meet his friends, triggering a conversation with her other friend, Hwa Ok ([Kim So Young]), about the upsetting incident.
Fast forward to the present, Gong Ju strolls down the school corridor and glimpses a group of girls rehearsing an a cappella song. When the group leader, Eun Hee ([Jung In Sung]), catches sight of her, she quickly walks away. During swimming lessons, they eventually meet again, and Eun Hee attempts to befriend Gong Ju, having admired her singing in the locker room. She invites her to join their a cappella group, but Gong Ju gently declines, instead borrowing Eun Hee's phone to locate her mother. After they take a bus together, Gong Ju faces rejection once more when her mother dismisses her. This encounter deeply unsettles her, prompting her to confront Eun Hee and recall Hwa Ok fondly. Overcome by anger, she later storms into her mother's boyfriend's shop, engaging in a reckless act of defiance before fleeing the scene.
Gong Ju finds solace in an abandoned music room at school, where she sings and unknowingly attracts Eun Hee and her friends' attention. Gradually, she opens up to them, sharing her dream of swimming at least 25 meters. However, her friends decide to upload a video featuring her singing online in hopes of starting a fan club, a plan that dramatically backfires when Gong Ju, furious at the exposure, demands it be removed, leading to an argument.
Their troubles escalate when irate parents storm into the classroom, demanding accountability from Gong Ju, whom they blame for their sons' imprisonment. In distress, she seeks refuge in an empty room.
The haunting past resurfaces as a flashback reveals the traumatic experience Hwa Ok endured, depicting Gong Ju's house turned into a scene of horror as she and her friend are drugged and gang-raped during a reckless party. The aftermath is devastating, marked by Hwa Ok’s tragic suicide by jumping off a bridge.
Returning to the present, Gong Ju meets with the principal, who is shocked by the revelations of her troubled past. Mrs. Cho's boyfriend questions her innocence, urging her to leave, but she ultimately chooses to distance herself, revealing that Hwa Ok was pregnant and that she regrettably did not intervene to help.
Meanwhile, Gong Ju's friends uncover the truth about the events that transpired and rally to support her, sending encouraging messages, affirming that she is not to blame. The horrific video of Hwa Ok’s assault leaves Eun Hee and her friends in disbelief. However, in her moment of despair, Gong Ju walks toward the edge of a bridge and jumps into the water. Underwater, a shadowy figure swims away gracefully, symbolizing a yearning for freedom. A voice asks her why she longs to swim, to which she replies, >“In case I want to start over because my mind could change.” As her friends chant her name, the scene fades with a chilling finality as she sinks into the depths.
