Due to financial problems, the boarding-school of Saint-Loup is on the verge of closing its doors. In desperation, Jacquelin, the headmaster, has the idea to invite former students to a fund-raising reunion. Among the alumni, three men whose common point is to have been in love at the time with the headmaster's niece. One is a banker, the second a priest and the last one a globetrotter. All of a sudden, the peace of the assembly is disturbed: Jacquelin's niece has just been murdered.

M. Jacquelin, directeur du collège de Saint-Loup, fait ses adieux aux enfants. L'établissement va devoir fermer, faute d'argent. Il profite de ses vacances pour inviter ses anciens élèves, onze ans après leur bachot, afin qu'ils lui viennent en aide. Parmi ceux-ci se trouvent trois amis jadis inséparables de Saint-Loup : Paul Forestier, curé, Jean Laclaux, banquier et Charles Merlin, globe-trotter.