The Comeuppance(1970)

Philippe, Théo and Bob share the same cell in the prison of La Santé. Philippe, a young student, known as "Sciences-Po", is planning revenge against those who sent him to prison. He offers his two fellow prisoners, seasoned mobsters, to help him recover an important document from a diamond dealer in Le Havre. This paper would rehabilitate his father, his two comrades being able to keep the diamonds contained in the safe... But the coup does not go as planned.