The movie opens with Johnny Barrett pitching a story to his editor about an unsolved murder at a mental institution. His editor shows no interest in the story until Barrett mentions that if he were to go undercover as a patient, he could solve the murder and win a Pulitzer Prize. His editor agrees to the plan, and Barrett convinces a psychiatrist, Dr. Fong, to commit him to the institution by faking symptoms of mental illness.
Once inside the hospital, Barrett is confronted with the harsh reality of life on the inside and the constant danger that comes with being among the mentally ill. The other patients are dangerous and unpredictable, and Barrett struggles to maintain his cover and avoid being taken advantage of. He also faces challenges from the hospital staff, who are skeptical of his claims to be a journalist and seem more interested in using him as a guinea pig for their experimental treatments.
Despite these challenges, Barrett manages to make progress in his investigation and gets close to uncovering the truth about the murder. He also becomes involved in the personal lives of the patients, including a young woman named Cathy who he befriends and tries to help.
As Barrett gets deeper into the investigation, he begins to experience hallucinations and delusions that blur the lines between reality and fantasy. He becomes increasingly unstable, and his girlfriend, who has been helping him from the outside, becomes worried about his mental state.
In the final moments of the film, Barrett confronts the killer and is nearly killed in the process. He is eventually rescued by Dr. Fong and his girlfriend, but it becomes clear that he has suffered permanent psychological damage as a result of his time in the hospital.
The film is a classic example of film noir, and is celebrated for its intense psychological themes and stylish visual style. It has been praised for its social commentary on issues of mental illness and the treatment of patients in psychiatric hospitals, and its exploration of the corruption of the media and the pursuit of fame and recognition at all costs.