Middle-aged Lucas Corso is a tired and cynical book detective, a mercenary hired to hunt down rare editions for wealthy and unscrupulous clients. When a well-known bibliophile is found hanged, leaving behind part of the original manuscript of Alexandre Dumas' The Three Musketeers, Corso is brought in to authenticate the fragment. Soon the unsuspecting Corso is drawn into a swirling plot involving devil worship, occult practices, and swashbuckling derring-do among a cast of characters bearing a suspicious resemblance to those of Dumas' masterpiece.