Six years after her father David’s death, Emma Grossman, now 15, lives with her aunt Angela Grossman, Angela’s husband Robert Costa, and their baby Cade. She projects a calm, ordinary teenager life—school, friends, and dance—while quietly masking a dangerous, calculating streak beneath the surface. Emma struggles with Robert’s presence in the home, and tension climbs when Robert notices his camping knife is missing from its usual place.
One day Emma tries to lure Cade into the pool, but Angela intervenes just in time to save him. At school, she discovers a boarding school brochure and learns that Robert intends to send her away to St. Crispin’s. A new student, Kat Sandburg, arrives and immediately clashes with Emma, fueling a rivalry that shadows Emma’s days. When Emma’s deception is exposed—Robert finds his missing knife in Emma’s drawer—he proceeds to arrange the transfer to the boarding school, pushing Emma toward a breaking point. She erupts with a meltdown and secretly spies on Robert, sharpening her grip on the situation in the house.
Emma meets with her therapist, who urges her to confront her own problems rather than rely on manipulation. She pretends to make peace with Robert, but that night she sabotages his car jack, causing it to topple and injure him. She walks away, whistling as if nothing happened. A sleepover with Kat turns tense when Kat needles Emma during a game of “Never Have I Ever,” and Emma is later rushed to the hospital to see Robert. While comforting Angela, she retrieves Robert’s knife, adding another layer to the mounting paranoia and danger in their home. Meanwhile, Robert wakes in the hospital and writes Emma’s name, signaling that the past is far from resolved.
The night before the dance captain election, Emma executes a calculated plan: she kidnaps classmate and rival Steph’s dog. The next day, Steph is absent due to the dog’s death, but she still wins the captaincy, sending Emma into a fierce fury. The emotional weight of the moment leaves Emma in turmoil, and she learns that Robert will be back in a week, triggering yet another meltdown.
Robert returns home in a wheelchair with a home nurse, and he doesn’t remember the night’s events. At school, Emma and Kat argue, ending with Kat slapping Emma. Emma then finds Steph’s seizure medication and steals it, a move that foreshadows the danger to come. Angela receives CCTV footage showing Steph’s dog being taken by Emma, a discovery that destabilizes the family’s fragile balance. Before Angela can process it, she learns that Steph has died from a seizure triggered by Emma. Angela confronts Emma, who denies any involvement with Steph or her dog.
Later, Emma stages a scene to steal Robert’s painkillers. Outside with him, she pretends to call Nathan, a classmate she admires. When Robert harms himself and the home nurse wheels him inside, Emma begins to whistle, triggering his memory. He gives Emma an ultimatum: go to St. Crispin’s or risk imprisonment. Angela discovers Emma has antisocial personality disorder, a truth that reshapes the family’s understanding of her. That night, Emma takes a psychopath test and is pleased with the result.
Angela turns to family friend Brian to ask about the night he shot her brother David, and Brian’s memory reveals a chilling comment about Emma that helps Angela see the true danger. Emma then steals the home nurse’s phone, unplugs the landline, and blocks Angela’s calls. She invites Kat over, promising to tell her everything, but then drugs Kat’s hot chocolate and confesses to her crimes. As Kat dies, Emma calls 911 and frames Kat for an attack and arson.
Robert hears Cade crying and climbs upstairs, only to discover a recording in the crib. Emma appears, taunts him, and starts a fire with gasoline. Angela returns and rushes inside to save Robert, but both die from smoke inhalation. Emma carries Cade to safety and feigns innocence as arriving firefighters, with Nathan offering comfort, unaware of her guilt. In the aftermath, Emma convinces a social worker to keep her and Cade together. Alone at last, she smiles wickedly at the camera, leaving a chilling sense of what she is capable of.
