King Arthur's legendary sword Excalibur had extraordinary powers in battle, but its enchanted scabbard was far more important than the blade: no weapons could harm its bearer, whom it made invulnerable.
Early in King Arthur's reign his evil half-sister, the witch Morgana Le Fay, stole the priceless scabbard and pretended to have lost it. In Knight for a Day it reappears almost thirty years later in the hands of her villainous son: the traitor Sir Mordred. Because of it people think he's invincible and will win, and so his rebellion gains opportunistic supporters and becomes a big threat.
King Arthur makes Henry an honorary Knight of the Round Table so he will be eligible to embark on a perilous quest to recover the precious scabbard, in accordance with Merlin's prediction. Each of his birthday presents can help Henry in his task, but above all he needs to be resourceful, cunning and brave if he is to succeed. Merlin instructs him how to invoke the magic at a precise place and time to return home to his family and friends.