Willy Ferrière is a gambler living beyond his means, and his mistress is as greedy as he's dead broke. One day, he says out of loud in a a Montparnasse café that he would give 100,000 francs to get rid of his wealthy aunt so he could claim his inheritance. Someone secretly lets Willy know it's a deal. The old lady is murdered, and a low-life criminal is manipulated to be the perfect suspect. But Superintendant Maigret feels something is wrong.