After residents of Thurley Grange are poisoned by arsenic in a box of chocolates, owner Lois Hargreaves hires Tommy and Tuppence to investigate the incident. Before they can arrive on the scene another poisoning occurs with deadly results leaving Tommy and Tuppence the task of discovering the murderer among the remaining suspects.

Tuppence's curiosity about a cryptic message in a newspaper personal column leads her to suspect a romantic rendezvous. Attempting to prove her theory correct she persuades Tommy to attend the Three Arts Ball and later to visit a cafe suspected to be a meeting point. Hearing a cry from an adjoining room, Tuppence investigates and discovers Lady Merivale. Before dying Lady Merivale whispers ""Bingo did it"" Captain Bingo Hale, Lady Merivale's lover, is arrested, which leads the Beresford's to go from accusers to trying to prove Captain Hale's innocence.

Tommy and Tuppence have failed to crack a robbery, and they are sitting in a smart hotel drowning their sorrows in cocktails. Suddenly, they have a murder mystery on their hands.

Monty Jones hires the Beresfords to help him win a bet and break a concocted alibi by beautiful Una Drake, who has agreed to marry him if he can.

Inspector Marriott of Scotland Yard needs Tommy and Tuppence's help to defeat a gang of forgers who appear to move in high society. He suspects that the exclusive Python Club is at the centre of the fraud, so the Beresfords go there armed with money to gamble...