Two million dollars have vanished into thin air. Lemmy Caution, an American agent, is assigned by the FBI to investigate in Casablanca. He soon finds out about a gang of gold smugglers, whose boss might be Saltierra, but Saltierra's mistress, Carlotta de la Rue, nicknamed "La môme vert de gris" stands in his way, at times helping Saltierra at other times helping him. Only when her brother is gunned down by the gang does she choose her side, Lemmy of course.