As a young boy, the quiet and troubled Luke witnesses the harrowing aftermath of a mass shooting at a local coffee shop. Amidst the chaos, he encounters another boy named Daniel, portrayed by Patrick Schwarzenegger, who exudes charm and confidence and quickly becomes Luke's friend. Despite his vibrant presence, Daniel is invisible to adults like Luke’s mother, Claire (Mary Stuart Masterson). Their friendship blossoms, providing Luke with a sense of companionship to help him navigate through his parents' divorce.
However, their bond takes a dark turn when Daniel persuades Luke to mix an entire bottle of Claire's psychiatric medication into a smoothie, claiming it will grant her superpowers. Tragically, this results in a near-fatal poisoning incident, leading Claire to insist that Luke send Daniel away. Luke is then tasked with symbolically confining Daniel within her mother’s old dollhouse.
Years pass, and a now college-aged Luke grapples with intense anxiety regarding his future, social interactions, and the burden of caring for his mother, who suffers from paranoid delusions and a distorted self-image. Confiding in his therapist, Dr. Braun (Chukwudi Iwuji), Luke expresses his fear of becoming like his mother. One fateful night, while visiting his childhood home, he unlocks the dollhouse and to his surprise, Daniel re-emerges but as an adult.
Initially, Daniel's influence seems harmless as he aids Luke in preventing Claire's suicide, achieving academic success, and even fostering a romance with artist Cassie (Sasha Lane). However, Daniel's behavior soon turns hostile as he begins to react violently when Luke defies him. When Luke hesitates to engage with a psychology student named Sophie (Hannah Marks) during a date, Daniel takes control of his body, leading to a chaotic encounter that results in Luke being expelled from campus. Doubting his own sanity, Luke fears he may be developing schizophrenia and attempts to banish Daniel back into the dollhouse, but his efforts prove futile.
As Luke's mental state deteriorates, he becomes convinced that Daniel is fully usurping his identity during his sleep. A visit to John Thigpen’s father, the perpetrator of the original shooting, reveals unsettling similarities between their experiences, deepening Luke’s understanding that Daniel is a supernatural being rather than just an imaginary friend.
In a desperate move, Dr. Braun makes a late-night visit hoping to sever the ties between Luke and Daniel, tragically resulting in Luke’s consciousness being trapped within the dollhouse while Daniel takes over his form, culminating in the murder of Braun. Targeting Cassie next, Daniel reveals his true nature, calling himself "a traveler," and pursuing her to the rooftop where she desperately pleads for Luke to return to reality. Hearing her voice, Luke gathers the strength to escape the confines of the dollhouse.
In their final confrontation, Daniel asserts that he has aided countless souls throughout history, claiming none have been worthy of his help. Luke, aware of Daniel's true parasitic nature, realizes they can never be entirely separated. To protect Cassie, he makes the ultimate sacrifice, jumping from the rooftop while ensuring she is safe. Cassie lies down beside his lifeless body, and in the shadows, Daniel transforms back to his monstrous self, suggesting he is on the hunt for a new host.
