Gabin plays Lequévic, an elderly deep-sea diver, who, while going about his underwater business, discovers the corpse of a young woman. It turns out that the body has been entombed by the unhinged leader of a criminal gang. Lequévic hopes to bring the criminal to justice and to save heroine Martine (Andrée Debar) from a similar fate. If Lequévic had harbored any romantic hopes for Martine, however, these are scuttled by the presence of handsome Michel (Henri Vidal).