The 1963 horror film Diary of a Madman follows Simon Cordier, played by Vincent Price, as he investigates a series of strange events that begin to occur after he acquires an antique trunk. The trunk contains the belongings of an executed murderer, including a notebook that details the man's encounters with a malevolent entity known as the "horla."
Soon after, Cordier begins to experience strange and terrifying occurrences, including objects moving seemingly on their own and mysterious writing appearing on surfaces. Eventually, he realizes that he has become the vessel for the horla, which is capable of possessing his body and controlling his actions.
Cordier seeks help from a doctor and a priest, both of whom are skeptical of his claims. He also meets a young woman named Odette, with whom he begins a romantic relationship. However, as the horla continues to exert its control over him, he becomes increasingly erratic and dangerous.
The film culminates in a showdown between Cordier and the horla, with tragic consequences. Diary of a Madman is based on the short story "The Horla" by French author Guy de Maupassant and is known for its eerie atmosphere and Price's captivating performance.