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Ben Shaw (Joshua Jackson) and his new bride, Jane (Rachael Taylor), leave New York for Tokyo, Japan, where Ben has a job as a photographer. While traveling, Jane hits a girl in the middle of the wilderness, running over her. They find no body or even a trace of blood and decide to leave, thinking the victim was all right. They later start to find mysterious lights in their photos, which are later identified as spirit photography by Ben's assistant, Seiko Nakamura (Maya Hazen). Jane begins to have eerie dreams and visions as if they are trying to tell her something, and senses a haunting presence stalking them. Ben begins to complain of severe shoulder pain, and his friends begin to comment he's looking bent and hunched over, though the doctor can find no cause. Seiko takes Jane to her ex-boyfriend, Ritsuo (James Kyson Lee), whose career is to investigate paranormal activities, and he tells them that the lights are spirits, manifestations of intense emotions trying to communicate. At a subway station, Jane spots the ghostly presence of the girl she hit, causing her to believe that she killed the girl. Ben later also has a similar terrifying encounter. They go to a medium, Murase; however, Ben refuses to translate what Murase says, claiming he is a fraud.
Jane decides to visit the office building in one of the photos. She takes pictures and encounters the yÅ«rei, and learns that the girl's name was Megumi Tanaka and that Ben knew her. When she confronts Ben about it, he admits that he and Megumi were once involved in a relationship, but that after the death of her father, she became very obsessive and clingy, and eventually he dumped her, with help from his two friends.
Ben's friends, Adam (John Hensley) and Bruno (David Denman), are killed by Megumi. After witnessing Bruno's death, Ben wants to leave but Jane hands Ben their wedding photo, which shows a distorted picture of Megumi. They realize she's been with them the entire time and go to Megumi's home, only to find her decayed body. She had committed suicide with potassium cyanide long before the car impact and thus they had actually first encountered her as a ghost at that time.
That night, Ben is tortured by Megumi. Jane screams at Megumi to leave them alone; Megumi stops with a brief sinister laugh, leaving Ben alive. After Megumi's funeral, Ben and Jane return to New York, thinking it's all over. However, Jane finds some recent photos in an envelope which show Megumi. With Megumi's clues, Jane finds a camera in a trunk and uploads the memory card into the laptop. There, Jane sees photos taken by Ben, showing Adam and Bruno raping Megumi before her suicide, while Ben does nothing but watch them doing the deed.
Ben returns home, where he tries to explain to a distraught Jane that he felt it was the only way to drive Megumi away. They had planned on using the pictures as blackmail against Megumi if she didn't leave him alone but it turned into rape. This explains why Megumi murdered Adam and Bruno, as Ben knew this was about revenge against all three of them. Believing that Megumi was trying to warn her, and disgusted by Ben's past actions, Jane concludes that she can't spend her life with someone like Ben so she leaves him. Ben tries to stop her, but Megumi locks the door and doesn't let him.
Driven mad by Megumi, Ben throws the camera across the room. It takes a picture of him, showing Megumi sitting astride his shoulders. Remembering his shoulder pains and the hospital where a nurse weighed Ben and it showed the weight of two people, Ben realizes that Megumi has been with him all along since her suicide without his knowledge. Horrified, and in an effort to rid himself of her, he electrocutes himself. He is rendered completely catatonic and sent to a mental institution, where he is shown sitting slumped over on the edge of his bed. The last scene shows Megumi still draped over his back.

Question: Who locks the door, preventing Ben from leaving?
Answer: Megumi