Crispin Guest, returning home after a late night, sees a body hurtling from the uppermost reaches of the London Bridge. Guest's attempted rescue fails, however, and the man—an armourer with a shop on the bridge—is dead. While whispers in the street claim that it was a suicide, Guest—known in certain London circles as The Tracker for his skill in solving puzzles—is unconvinced.

What Guest uncovers is that the armourer had promised Sir Thomas Saunfayl, a friend from Guest's former life, that he would provide him something that would make him unbeatable in battle, something for which he'd paid a small fortune. Sir Thomas believes that the item was in fact the Spear of Longinus - the spear that pierced the side of Christ on the cross—which is believed to make those who possess it invincible.

Complicating matters is another old friend, Geoffrey Chaucer, who suddenly comes to London and is anxious to help Guest find the missing spear, about which he seems to know a bit too much. With various forces anxious to find the spear, the life of Sir Thomas in danger and perhaps the very safety of England hangs in the balance, Guest and his apprentice Jack Tucker must navigate some very perilous waters if they are to survive.