In late fourteenth-century London, disgraced former knight Crispin Guest spies a body hurtling from the uppermost reaches of London Bridge. Crispin's attempted rescue fails, however, and the man - an armorer with a shop on the bridge - dies. While whispers in the street claim that it was a suicide, Crispin is unconvinced. He discovers that the armorer had promised Sir Thomas Saunfayl something that would make him unbeatable in battle. Sir Thomas, a friend from Crispin's former life, 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 is suddenly eager to help Crispin find the missing spear...and locate Sir Thomas, who must face a trial by combat to determine whether he lives or dies. Desperate to help, Crispin, along with his faithful apprentice, Jack Tucker, hunt for the spear while dodging the various powerful factions determined to have it for themselves. Through danger and trials, Crispin finds that the safety of England may have landed solely in his hands.