The story centers on a harsh physician (Paul Muller) who finds out his spouse (Barbara Steele) is involved with someone else, leading him to inflict fatal suffering upon both individuals before utilizing their remains as fertilizer for a plant. His scheme takes another turn when he learns that his wife has bequeathed her substantial wealth to her unstable sister—who is also portrayed by Barbara Steele—prompting him to enter into marriage with intentions of having her declared mentally unfit so he can inherit the money. Predictably, his clever strategy doesn't unfold exactly as envisioned. With the arrival of psychiatrist "Dr. Joyce" (Laurence Clift) at his estate, the specters of those he harmed materialize, and chaos ensues—including instances of blood transfusions and even electrocution! The script is noticeably weak, and I felt Ennio Morricone’s forceful organ score was excessive and ultimately detracted from what is otherwise a pleasant, low-budget thriller that develops into a reasonably satisfying conclusion during the final fifteen minutes.

Source on TMDB