Ona a side trip to France, detective Adrian Monk takes a tour through the historic underground world of the Parisian sewers, where he encounters a catacombs filled with ancient human remains, including one much more recent skull bearing evidence of murder.
Reverting to his OCD habits when his therapist, Dr. Kroker, heads off for a conference in Germany, an anxious Adrian Monk follows his shrink to Europe and comes face to face with a man who might be the person responsible for his wife's death.