Tom Penderel (Tom Poston), an American working in London as a car salesman, is asked by his flatmate Caspar Femm (Peter Bull), who's convinced that his life is in danger, to deliver a new convertible to his ancestral home in the Dartmoor countryside.
There, Tom meets the rest of Caspar's eccentric family — including his twin brother, Jasper (Bull); his parents, Roderick (Robert Morley) and Agatha (Joyce Grenfell); his uncle, Potiphar (Mervyn Johns); his nieces, Cecily (Janette Scott) and Morgana (Fenella Fielding); and Morgana's father, Morgan (Danny Green) — and gets caught up in a murder mystery centered around the Femms' vast family fortune.
This film has examples of the following tropes:
- Animated Credits Opening: Featuring background art by none other than Charles Addams.
- The Ark: Potiphar is convinced that another biblical flood is imminent, and has built an ark, complete with animals, in the backyard of the Femm house.
- Chairman of the Brawl: Tom tries to deter Morgan by breaking a chair on his head, to no avail.
- Hoist by His Own Petard: Cecily is blown up with one of the bombs she planned to kill everyone with.
- Closed Circle: The raging storm outside forces Tom to stay in the Femm household, to his dismay.
- Hollywood Acid: For his washup, Tom receives a bowl of acid that only reveals its true colors when he accidentally dips his tie into it.
- Light Feminine and Dark Feminine: Cecily and Morgana, respectively.
- On One Condition: If a member of the Femm family wants to keep their share of the fortune, they must live in the house and be present for the daily gathering or they'll lose it.
- Overprotective Dad: Morgana's father Morgan is a silent and violent man who is ready beat the crap out of anyone who dares to touch his daughter. Since Morgana's taste for men is insatiable, he's had to do plenty.
- Red Herring:
- The possibility of Tom being related to the Femms is only just that, a possibility.
- The violent Morgan and domineering Rodrick are posited as prime suspects.
- The Reveal: The seemingly sweet and innocent Cecily has been killing her relatives.
- Shovel Strike: As Morgan approaches Tom with murder on his mind, Morgana whacks him in the head with a shovel.
- Trap Door: If one uses the doorbell instead of the knocker on the Femm household's front door, he'll find himself in the basement.
- The Voiceless: Morgan is mostly silent, preferring to let his fists to do the talking.