During the Nazi occupation of Paris, the deranged doctor Petiot (Michel Serrault) abuses the trust implied by his profession to "help" frightened Jewish citizens. By day, he conducts his clinic and supports his family with a kindly obsession. By night, he leads his victims from a metro-station rendezvous to his apartment, their worldly possessions dragged in a trailer behind his bicycle. He then administers a "vaccine" and locks the now poisoned refugee in a room to face an agonizing death alone.

The doctor takes the possessions of his victims, and dismembers and incinerates their corpses in a makeshift crematorium in his basement. In March 1944, the nauseating black smoke betrays his activities; however, the now notorious doctor vanishes, abandoning his wife and son. Following the war, he is living incognito as a soldier pursuing war criminals and collaborators. But he is identified by his fascination with the Petiot case and his handwriting. In the final scene, dozens of people stand at a long table silently sorting through clothing, jewelry, books, seeking belongings of their loved ones who became the doctor's victims.