Pharmacist Yan Lazarrec believes in saving the ecology. However, his commitment takes on worrying proportions when he believes his mission is to remove pollutants of any kind from the Earth. Yan drowns an industrialist in an oil pond, asphyxiates a tobacco tycoon with cigarettes and poisons a director of a cosmetics company. Police officer François Barrier is on the track of this serial killer, but he is conflicted because he also has a passion for ecology. He becomes acquainted with Yan at a conference and begins to sympathize with him, without knowing his true identity.