Lucius, 73 years old, lives alone and modestly. One day, he makes the acquaintance of Lahire, who proposes to improve his daily leanness by participating in clandestine boxing fights. By necessity and because he knows that his days are numbered, Lucius accepts the proposal. Fascinated by this universe, he takes pleasure in this new existence. This strange experience will completely upset his life, in the eyes of Mona, the only person to whom he is close.