Echo, a high school freshman, struggles with the profound grief of losing her older sister, Zoë, who was tragically murdered a year ago. On her first day back at school, Echo feels the weight of her loss as many students gawk at her or whisper behind her back. Despite this, some, including her crush Parker, approach with sympathy and condolences. Compounding Echo's pain, her mother has turned to antidepressants while her father works excessive hours, leading to frequent visits with a family therapist that Echo finds far from helpful.
During lunch with her best friend Abby, Echo spots Zoë's boyfriend, Marc. Abby informs Echo that Marc was a prime suspect in Zoë's murder after a suspected internet predator confessed. Although the true killer is behind bars, a part of Echo can't shake the feeling that Marc is somehow involved. One day, she discovers Marc's new car and notices Zoë's diary in the passenger seat. Driven by curiosity and a desperate need for answers, she smashes a window to retrieve it.
As Echo dives into the diary, she uncovers the history of Zoë and Marc's relationship, discovering they had met at a pool party over the summer. She learns that Marc, who is in recovery from addiction, had a tumultuous past involving his ex-girlfriend, but claims he has changed. Despite his assurances, Echo remains skeptical.
When Echo and Abby attend a party thrown by Chess, it turns out to be less exciting than anticipated. However, Echo finds a moment of connection with Parker, leading to an unexpected kiss after a conversation about Zoë. That night, Echo goes to her first school dance. Amidst the emotional chaos, she encounters Carley, Zoë's best friend, who encourages her to drink and experiment with ecstasy. In a hallucinatory bathroom moment, Echo sees an apparition of her sister, who implores her to stop pretending everything is okay.
Later, in her drugged state, Carley confuses Echo for Zoë, expressing regret about something involving an unnamed male and a video. This adds to Echo's growing disillusionment, especially after a flashback reveals Marc being questioned by police regarding the night of Zoë's death.
As Echo reads more of the diary, it unveils darker secrets, including a pivotal night where Zoë went out with Carley and, despite her hesitations, ended up traumatized after a meeting with a man named Jason. After being shown a photo of him, Zoë had warned Echo to stay away if she ever saw him.
Determined to seek the truth, Echo attends a party at Theresa's home dressed in Zoë's clothing, stirring rumors among her peers. There, she encounters Jason, affirming her intuition about his connection to her sister’s past.
As Echo ventures deeper into the mystery, she organizes a plan to confront Jason and uncover the truth behind the video that tied Zoë to her tragic fate. With the help of Marc and Parker, she finds herself breaking into Jason’s house, which ends in chaos when Marc’s desperate act leads to a confrontation revealing heinous crimes against several girls, including Zoë.
In a climactic series of events, Echo, with Parker's help, manages to escape a dire situation and ensures that justice is served against Jason and his accomplice. The shocking truth is brought to light, leading to their arrest on multiple charges, including the production of child pornography and murder.
As Echo comes to a close with Zoë’s diary, she reflects in silence, apologizing for not knowing her sister better and cherishing the loving memories they shared. In a poignant moment of healing, Echo begins penning her own thoughts in the new diary Zoë gifted her, sitting at a memorial bench created in her sister’s honor. Thus, she takes the first steps toward moving forward, not forgetting but embracing her sister's legacy.
