The husband of Monk's former assistant, Sharona, has been arrested for murder. Now back in San Francisco, she's ready to reclaim her place in Monk's life — much to the chagrin of his current assistant, Natalie. While Monk tries to maintain a delicate balance between the two women, he discovers a few unsettling snags in the case against Sharona's husband, and may be up against a killer who not only understands him, but is one step ahead.

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

Captain Leland Stottlemayer is faced with budget cuts and trims Monk's services as an expense. Though he insists it's nothing personal, when Monk's offers to help for no charge are declined, he and Natalie suspect otherwise. But Monk is too compulsive to stop investigating. So when Stottlemayer is arrested for the killing of another cop with overwhelming evidence against him, he must turn to the only detective who can prove his innocence.