Plot
Six years after her father David's death, 15-year-old Emma Grossman lives with her aunt Angela, Angela’s husband Robert, and their baby Cade. Still unsuspected of her crimes, Emma maintains the façade of a normal teenager, hiding her psychopathy beneath school, friends, and dance. In a flash-forward, she is seen killing a dog, while newspaper clippings about her past murders appear.
Three weeks earlier, Emma struggles with Robert’s presence in the household. During breakfast, she mentions upcoming dance captain elections, expecting to beat her friend Steph. Robert notes his camping knife is missing. Emma later tries to lure Cade into the pool, but Angela saves him.
At school, Emma finds a boarding school brochure and learns Robert wants to send her to St. Crispin’s. A new student, Kat, arrives and immediately clashes with Emma. At dance practice, Steph is praised while Emma is told to improve. Emma practices alone and rejects a ride home from classmate Nathan.
The next day, Steph invites Kat to sit with their group. Emma complies, but Kat declines. Later, Robert finds his missing knife in Emma’s drawer and arranges her transfer to St. Crispin’s. Emma has a meltdown and later spies on Robert.
Emma meets with her therapist, who advises her to solve her own problems. Emma pretends to make peace with Robert, but that night, she sabotages his car jack, causing it to fall on him and break his legs. During a sleepover, Kat needles Emma during a game of "Never Have I Ever", but the fun ends when Emma is taken to the hospital to see Robert. While comforting Angela, she retrieves Robert’s knife. Meanwhile, Robert wakes in the hospital and writes Emma’s name.
The night before the dance captain election, Emma kidnaps Steph’s dog. The next day, Steph is absent due to the dog’s murder. Steph still wins captain, sending Emma into a rage. She learns Robert returns in a week and has another meltdown.
Robert comes home in a wheelchair with a home nurse, Cora. Emma asks if he remembers his accident, and he says no. At school, she and Kat argue, ending with Kat slapping Emma. Emma finds Steph’s seizure medication and steals it. That night, she lies to Angela about Kat bullying her.
Angela receives CCTV footage of Steph’s dog being taken by Emma. Before she can process it, she hears Steph has died from a seizure, triggered by Emma. Angela confronts Emma, who denies any involvement.
Later, Emma stages a scene to steal Robert’s painkillers. Outside with him, she pretends to call Nathan. When Robert hurts himself, Cora wheels him inside. Emma’s whistle triggers his memory. He gives Emma an ultimatum: go to St. Crispin’s or face jail.
Angela discovers Emma has antisocial personality disorder. That night, Emma takes a psychopath test and is pleased with the result. Angela meets family friend Brian to ask about the night he shot her brother David. Brian recalls David making a chilling comment about Emma, and Angela finally understands.
A storm begins. Emma steals Cora’s phone, unplugs the landline, and blocks Angela’s calls. She invites Kat over, promising to tell her everything. Emma drugs Kat’s hot chocolate, then confesses to her crimes. As Kat dies, Emma calls 911 and frames her for an attack and arson.
Robert hears Cade crying and pushes himself upstairs, only to find a recording in the crib. Emma appears, taunts him, and starts a fire with gasoline. Angela returns and runs inside to save Robert. Both die from smoke inhalation. Emma carries Cade to safety and plays innocent for arriving firefighters, as Nathan comforts her, unaware of her guilt.
After the fire, Emma convinces a social worker to keep her and Cade together. Once alone, she smiles wickedly at the camera.
