Alain Marchal, wrongly accused of murdering his wife, escapes from prison the day before his execution. Before committing suicide, he contacts Monsieur Charles, a retired gangster, and asks him to investigate, find the real culprit and clear his name. Monsieur Charles accepts his request and along with his wife Ginette and two former accomplices, Paragraphe and Teddy Coffre fort, embarks on his counter-investigation.

Micheline, who fled to Paris with the man she is in love with, finds herself on her own when he abandons her. She lands in a reformatory from which she manages to escape.

When Monsieur Crauqual, the supervisor, is found hanging, all wonder why this rather rich person decided to kill himself. And when a pupil is found hanging too, François and his friends decide to investigate. They are certain there is a killer within the walls of Collège François 1er and determined to prevent him from going on running wild.