Mima Kirigoe, member of a J-pop idol group named "CHAM!", decides to leave the group to become a full-time actress. She gets stalked by a creepy-looking fan named Me-Mania, who is upset by her change from a clean-cut image. Following directions from a fan letter, Mima discovers a website called "Mima's Room" containing public diary entries written from her perspective, and which has her daily life and thoughts recorded in great detail. During her acting career, she is joined by manager and former pop-idol Rumi Hidaka, and her agent Tadokoro. Mima confides in Rumi about "Mima's Room", but is advised to ignore it.
Mima's first job is a minor role in a television detective drama called Double Bind, however, Tadokoro lobbies the producers of Double Bind, and succeeds in securing Mima a larger part that involves a rape scene. Despite Rumi's objections, Mima accepts the role, although this leaves her severely affected. On her way home, she sees her reflection dressed in her former idol outfit. She would claim she's "the real Mima". Between the ongoing stresses of filming Double Bind, her lingering regret over leaving CHAM!, her paranoia of being stalked, and her increasing obsession with "Mima's Room", Mima begins to suffer from psychosis: in particular, struggling to distinguish real life from her work in show business, and having repeated apparently unreal sightings of her former self.
Several people who had been involved in her acting are murdered. Mima finds evidence which makes her appear to be the prime suspect, and her mental instability makes her doubt her own memories and innocence, as she recalls brutally murdering photographer Murano. Mima manages to finish shooting Double Bind, the final scene of which reveals that her character killed and assumed the identity of her sister due to trauma-induced dissociative identity disorder. After the rest of the filming staff have left the studio, Me-Mania attempts to rape and kill her, acting on emailed instructions from "the real Mima" to "eliminate the impostor", but Mima knocks him unconscious with a hammer.
Mima is found backstage by Rumi and taken back to Rumi's home, only to discover that Rumi was the culprit behind "Mima's Room", the serial murders, and the folie à deux that manipulated and scapegoated Me-Mania. Rumi previously developed a second personality who believed herself to be the "real Mima", using information from Mima's confiding in her as the basis for "Mima's Room". Rumi's "Mima" personality chases Mima through the city to murder her. Mima incapacitates Rumi in self-defense and saves her from being killed by an oncoming truck.
Some time later, Mima is now a well-known actress and visits Rumi, who has been living in a mental facility for some time. The doctor in charge says that Rumi still believes she is a pop idol most of the time. Mima says she's learned a lot from her experience thanks to Rumi. As Mima leaves the hospital, she overhears two nurses talking about her, a famous actress, before they think she is a Mima lookalike, as the real Mima Kirigoe would supposedly have no reason to visit a mental institution. As Mima enters her car, she smiles at herself in the rear-view mirror before declaring, "No, I'm the real thing."