What could possibly be the connection between an allegedly authentic manuscript of a chapter from Dumas' The Three Musketeers and a 17th century occult text, a manual for invoking the devil, only one copy of which is believed to have survived after its was publisher was burnt at the stake? Gin-swilling book-hunter Lucas Corso is hired to authenticate the Dumas manuscript. At the same time, Varo Borja, the owner of an occult text, The Nine Doors, persuades him to investigate two further editions of the book known to be in collections in Sintra and Paris. As Corso digs into the mystery, the twilight world of antiquarian bookselling, a milieu of ruthless predators, unfolds. Corso finds himself sharing a stage with a cast of characters who seem to have walked straight off the pages of a Dumas novel. And Corso appears to have been assigned the role of D'Artagnan.