Plot summary

As a child, shy and troubled Luke witnesses the aftermath of a mass shooting at a neighborhood coffee shop. He meets another boy among the onlookers at the scene: cool and confident Daniel, who invites him to play and quickly becomes his friend. Although adults such as Luke's mother Claire cannot see Daniel, he appears physically real to Luke. The boys become close playmates, and their connection helps Luke cope with his parents' divorce. Their friendship comes to an abrupt end when Daniel convinces Luke that blending an entire bottle of Claire's psychiatric medication into a smoothie will give her superpowers. Instead, it results in a near-fatal poisoning. Claire convinces Luke to send Daniel away by symbolically locking him in her mother's old dollhouse. Years later, the college-aged Luke is paralyzed by anxiety over his future, his social life and his responsibility to his mother, who struggles with paranoid delusions and a hatred of her own reflection. He confides to his therapist, Dr. Cornelius Braun, that he is afraid he will eventually become just like her. One night, while sleeping over at his childhood home, Luke unlocks the dollhouse. Daniel reappears as an adult. His influence initially appears benign as he helps Luke thwart Claire's suicide attempt, succeed in school, and begin a romance with an artist named Cassie. However, he quickly starts exhibiting aggressive behavior, becoming enraged when Luke doesn't obey him. When Luke won't have sex with a psychology student named Sophie on a date, Daniel forcibly takes over his body, has rough sex with Sophie and attacks Luke's roommate. Luke is banned from campus as a result and begins to question his own sanity, believing he may have schizophrenia. He attempts to banish Daniel back to the dollhouse but is unsuccessful. Luke becomes increasingly unstable, convinced that Daniel is taking over his body while he sleeps. He visits the father of John Thigpen, the shooter whose crime began the film, and learns John also had an invisible friend named Daniel. Luke begins to realize Daniel is a supernatural entity, not an imaginary friend. Dr. Braun makes a late-night house call in an attempt to separate Luke and Daniel, but only succeeds in allowing Daniel to take over Luke's body and banish Luke's consciousness to the dollhouse. Daniel then kills Braun. He decides his next target will be Cassie. Daniel reveals his true nature to Cassie, describing himself as "a traveler," before chasing her to the rooftop of her apartment building. Cassie begs Luke to come back to reality. Hearing her from inside the dollhouse, he summons the will to escape. In a final confrontation with Luke, Daniel claims he has helped people for centuries, but none of them have deserved his help. Luke calls him a parasite. Luke realizes he and Daniel can never truly be separated. After ensuring Cassie has escaped to safety, he kills himself by jumping from the roof. Cassie lies down beside his body. Daniel, surrounded by an otherworldly darkness, reverts to his true monstrous form, implying he will now seek out a new host.