A country squire hires Poirot to investigate threats against his young son.
Marcus Waverly, an English country squire, calls on Poirot for help in dealing with anonymous letters threatening to kidnap his son, Johnnie. Hastings is deeply shocked by the thought of people going around kidnapping children, and it becomes a race against time to find out who the mystery letter writer is.
Of course, there is a lot more to the mystery than kidnap threats.