Maigret is in Rennes to reorganize their flying squad, and is called to Concarneau where Monsieur Mostaguen, a respectable citizen, has been shot dead while leaving the Admiral Café. Mostanguen was a member of a group that met regularly at the cafe, and at first here appears to be no reason for the crime. Circumstances point to him being in the wrong place at the wrong time.

Then another member of the group is wounded, one is found poisoned, and strychnine is discovered in others' drinks. Throw in a meek waitress, a ferocious vagrant, and a yellow dog, and Maigret has to dig deep into the past to find the motive — and the real murderer.