Madame Maigret telephones to alert her husband that a frightened young man has called at their home and will wait there to see Maigret at his midday meal. When Maigret returns home, he is annoyed to discover that his mysterious guest has left, and more disturbed that one of his prized revolvers has gone with him.

Monsieur Maigret is dismayed - but hardly surprised - when a corpse turns up in a railway trunk, shot by his own gun. His search for the elusive and unidentified visitor sends Maigret across the channel to London to investigate one murder... and prevent another.