Eleven-year-old Jon Whitcroft and new friend Ella summon the ghost of Sir William Longspee, who may be able to protect Jon from a group of ghosts that threatens him harm from the day he arrives at Salisbury Cathedral's boarding school. Includes historical notes.

Relates the adventures of Sir Gawain, the only undefeated knight in King Arthur's court, who eventually learns the value of friendship, courtliness, and courtesy after a challenge from the Green Knight.