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 ancient human bones. Monk picks up a skull and announces that this person was murdered - by a modern weapon made in the last few years. He thinks someone hid a recent murder victim among the old 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.