Crispin Guest, Tracker of London, is enjoying his ale in the Boar’s Tusk tavern – until a stranger leaves a mysterious wrapped bundle on his table, telling him, "You'll know what to do." Inside is an ancient leather-bound book written in an unrecognizable language. Accompanied by his apprentice, Jack Tucker, Crispin takes the unknown codex to a hidden rabbi, where they make a shocking discovery: it is the Gospel of Judas from the Holy Land, and its contents challenge the very doctrine of Christianity itself.

Crispin is soon drawn into a deadly maze involving murder, living saints, and lethal henchmen. Why was he given the blasphemous book, and what should he do with it? A series of horrific events confirm his fears that there are powerful men who want it – and who will stop at nothing to see it destroyed.