Five diverse, infamous private eyes, including Sam Diamond (Peter Falk), Sidney Wang (Peter Sellers) and Jessica Marbles (Elsa Lanchester), are invited to the mysterious Lionel Twain's castle for a dinner party. There, they are told that Twain plans an unsolvable murder in the house at midnight -- and he will pay $1 million to the one who determines the killer. But when Twain's blind butler, Bensonmum (Alec Guinness), dies long before the deadline, the stakes go up for the trapped sleuths.