Following the shocking discovery of his wife's lifeless body in the bathtub following her tragic suicide, Dr. David Callaway, portrayed by Robert De Niro, a psychologist residing in bustling New York City, decides it's time for a fresh start. He relocates to the serene and quiet landscape of upstate New York with his 10-year-old daughter, Emily, played by Dakota Fanning.
In their new surroundings, Emily befriends a seemingly imaginary playmate whom she names "Charlie." However, as their bond deepens, David grows increasingly concerned when he discovers their cat dead in the bathtub, a victim of "Charlie," according to Emily. Disturbing dreams plague David, revisiting the harrowing New Year's Eve party preceding his wife's death.
The arrival of a family friend, Katherine, brought to life by Famke Janssen, brings further complications when Emily reveals to David that both she and Charlie share a mischievous intent to provoke him. A neighboring couple, driven by the heartache of having lost their daughter to cancer, takes a peculiar interest in Emily, raising David's suspicions. His fears are compounded when he learns that the woman implies she has suffered emotional and potential physical abuse from her husband following their daughter's passing.
In an effort to foster healthy friendships for Emily, David meets a local woman named Elizabeth, played by Elisabeth Shue, and her niece, Amy. Despite David's good intentions in arranging a playdate, it takes a turn for the worse when Emily whimsically mutilates Amy's doll, causing the playdate to implode as Amy flees in fear. Meanwhile, a budding connection between David and Elizabeth seems promising. However, when Elizabeth attempts to make amends with Emily, pretending to engage with Charlie during a game of hide-and-seek, tragedy strikes. As Elizabeth investigates a closet, she is suddenly attacked and pushed to her death from a window.
After the police find her wrecked car near David's home, he interrogates Emily about the incident. She chillingly claims that Charlie instigated the death by shoving Elizabeth through the window and directs David to her body, which astonishingly is discovered in the bathroom filled with blood. Frantic and desperate for answers, David brandishes a knife and confronts a suspicious neighbor who has been developing a friendship with Emily. Misguidedly believing the neighbor to be Charlie, David reacts violently. The escalating situation prompts the neighbor to call the authorities.
As the fraying edges of reality blur, David realizes that despite spending hours in his study, the boxes from their move remain untouched. This revelation compels him to confront a terrifying truth: he suffers from a split personality, with Charlie being a manifestation of his own psyche. Underneath his gentle demeanor lies a darker, rage-filled identity responsible not only for his wife's death but for the façade of suicide that followed. Torn apart by the memories of the fateful New Year's Eve party, David recalls the unbearable moment he caught his wife in a compromised state with another man. In an effort to channel his rage, Charlie emerged.
As David becomes fully consumed by Charlie, matters take a lethal turn when he murders the local sheriff investigating the earlier incident. In desperation, Emily reaches out to Katherine for help.
Katherine rushes to the scene but falls victim to Charlie's wrath when he pushes her down the basement stairs. With a sinister intent to resume their deadly game of hide-and-seek, Charlie begins counting as Emily desperately seeks refuge. She cleverly manages to lock herself in her room; however, in a moment of vulnerability, Katherine seizes the sheriff's weapon and makes her way out to confront Charlie in the cave where Emily first encountered him.
In a gripping confrontation, Charlie deceives Katherine by masquerading as David, launching an assault when she drops her guard. Pleading for David to emerge and confront the monstrous split of himself, Katherine implores him. Charlie taunts her, revealing how David's self-awareness had ultimately allowed Charlie to take command. Just as Emily appears, appealing to Charlie to spare Katherine, her presence provides the crucial distraction Katherine needs to shoot Charlie fatally.
In the aftermath, as Emily prepares for a new school day under Katherine's care, an unsettling twist is unearthed in her artwork—a drawing depicting herself and Katherine with two heads—implying that the traumatic ordeal may have left her with the same fractured psyche that once plagued her father.
