In this thriller, Albert Finney stars as an innocent-looking but psychopathic killer named Danny, a Welsh bellboy who commits an axe murder near the home of Mrs. Bramson (Mona Washbourne), a well-to-do widow. Danny disposes of the body in a nearby lake and charms Mrs. Bramson and her maid, Dora (Sheila Hancock), into allowing him to stay with them. At Mrs. Bramson's home, Danny plays psychological games with Mrs. Bramson while seducing her daughter, Olivia (Susan Hampshire). As the authorities close in on Danny, he engages in strange rituals with the severed heads of his victims, which he keeps in a black hatbox.