On a return trip from Germany, Monk and Natalie stop over in Paris. They visit the catacombs, a vast, underground cavern in the center of the city lined with millions of human bones. Monk picks up a skull and announces that the person was murdered. Most of the bodies came from a battlefield, but Monk believes the wound that killed this man came from a modern weapon made in the last few years. He thinks someone hid a recent murder victim among the ancient bones. As Monk starts to investigate the crime, he uncovers another murder that involves a society of 'dumpster divers,' people who live commerce-free off the waste of society. It might just be the last straw in Monk's visit to Paris.