Hercule Poirot receives a letter announcing that something will happen in the town of Andover on June 21. At first he thinks of a joke, but then someone is killed on the announced date and place. A second letter arrives, predicting a murder on July 25. And now Poirot knows it's bittersweet. But how can he prevent someone else from dying? Based on 'The Alphabet Killer' by Agatha Christie.