Paul and Jessie Duncan, portrayed by Greg Kinnear and Rebecca Romijn, enjoy a blissful marital life with their beloved eight-year-old son, Adam, played by Cameron Bright. Tragically, their happiness is shattered when Adam dies in a car accident just a day after celebrating his birthday. In the aftermath of their devastating loss, the couple encounters Dr. Richard Wells, a former professor of Jessie’s, portrayed by Robert De Niro, who proposes an audacious solution: cloning Adam. Although initially hesitant, the Duncans agree to undergo this illegal procedure, involving a complete change of their identities.
As the new Adam grows up, everything seems promising until he reaches the age at which the original Adam met his untimely fate. On the night of his eighth birthday, he suffers from intense nightmares, which Dr. Wells reassures Paul are a common ordeal for boys his age. However, the situation takes a sinister turn as Adam II begins to experience these nightmares while awake, losing grip on his reality amidst a terrifying wave of visions.
These haunting visions consist of a tormented boy named Zachary, wandering alone through a school, where he is ridiculed by his peers. This bleak imagery is interspersed with horrific scenes of a school engulfed in flames and a mysterious woman brutally attacked with a hammer. Over time, these disturbing events impact Adam’s behavior, turning him into a bitter and troubled child who begins bullying his classmates.
One evening, a disturbing phone call from the parent of a missing child leads to an unsettling revelation: Adam had lied about his whereabouts on the day of the child's disappearance. Shortly thereafter, Paul and Jessie are confronted by the chilling sight of the boy's body being retrieved from the river. Consumed by suspicion, Paul grows increasingly anxious about Adam's involvement in these chilling events.
With Dr. Wells' guidance, Paul delves deeper into Adam's visions, uncovering the identity of the school in his dreams—Saint Pius—and the name of the bullied boy—Zachary Clark. This newfound clarity drives Paul on a quest to learn more about Zachary's troubled past, leading to the shocking discovery that Zachary's life ended tragically when he set fire to his home and killed his mother.
In a twist of fate, Paul learns that Zachary's father, a geneticist, is none other than Dr. Wells himself, who had combined Zachary’s damaged DNA with Adam's in a desperate attempt to resurrect his lost son. Faced with this realization, Paul rushes home, only to find Adam and Jessie in grave danger, as Zachary’s essence begins to take over Adam’s mind, pushing him towards violence.
In a frantic race against time, Paul manages to save Jessie just moments before Adam, under Zachary's influence, strikes. Although the immediate threat is vanquished and Adam appears back to normal, the family decides to relocate in search of peace. Initially, their new life seems idyllic with Adam once again showing signs of happiness. However, when left alone in his room, the nightmare returns—Zachary's decayed arm reaches out from the depths of a closet, drawing Adam into a shadowy grip, signaling that the battle for identity is far from over.
