The Adventure of Johnnie Waverly
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"The Adventure of Johnnie Waverly" is a short story by Agatha Christie, collected in Poirot's Early Cases. The third episode of the series Agatha Christie's Poirot was based on it.
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.