King Odin, of a neighbouring Kingdom, commissions the deadly asassin Myror to kill Prince Arthur, in order to take revenge for the fact Arthur killed Odin's son in a duel. Myror sets off for Camelot.
Meanwhile, Arthur realises, when practising for the jousting tournament, that he is being treated differently and allowed to win because he is a Prince. Arthur becomes keen to show that he can win with no special treatment. He concocts a plan to enter the tournament without people knowing who he really is. He tells Uther that he has heard word of a beast, conjured by magic, roaming the nearby woods. He describes the beast as "Having the body of a lion, the wings of an eagle, and the face of a bear" (much to Merlin's amusement). He tells his father that he will set off to hunt the beast in the morning, missing the tournament. The next morning he leaves the castle with a hunting party, but stays behind in the woods, telling his men not to let anyone know he has returned to Camelot. Merlin brings him some civilian clothes, and Arthur goes to hide in Gwen's house during the tournament.
Myror, lying in wait for the hunting party in the woods, discovers that Arthur is missing and goes to Camelot to try and find him. Uther is informed of Odin's plan, however Merlin is still ignorant of it.
The plan to allow Arthur to enter the tournament anonymously involves employing a farmer, who is unknown in Camelot, to play the knight "Sir William of Daira" when riding out and greeting the audience. The farmer will then enter a tent, and be replaced by Arthur wearing a helmet. The tournament goes well, with Arthur beating all opponents. He plans to reveal himself after the final, to experience his "moment of glory".
Gwen, after putting up with Arthur's arrogant attitude while he stays with her, finally explodes and tells him how rude he has been. Arthur realises that he has been behaving appallingly, and tells her he will make it up to her by cooking dinner. However, in reality he has Merlin procure two dinners from the castle. While Gwen and Arthur are eating, Myror has traced Arthur and is about to shoot him when he is distracted and nearly caught by the castle guards. Myror runs away before he can be caught.
Gwen discovers Arthur's deception after recognising the plates as belonging to the castle kitchens. She becomes angry with Arthur for lying to her, telling him she thought he had learnt some humility. He apologises and tells her he cares about her. Merlin bursts in, having discovered that there is an assassin in Camelot trying to kill Arthur.
The next day, while Gwen is getting Arthur ready for the tournament, she gives him a handkerchief 'For luck.' He kisses her and then, in haste, leaves.
Myror has discovered Arthur's participation in the tournament, and so he kills Arthur's opponent and takes his place. As he and Arthur ride up to each other, a hidden blade comes out of Myror's lance and injures Arthur. Merlin sees the blade, and discovers Arthur's opponent dead in his tent and Myror wearing his armour.
Arthur, despite his injuries, refuses to forfeit the match. Before Merlin can reach him and warn him, Arthur rides again, and Merlin is forced to incapacitate Myror by breaking a strap on Myror's saddle, distracting him so that Arthur can beat him and (though unintentionally) kill him. Arthur lets the farmer posing as 'Sir William' go to the crowd and take the glory. He says, 'Perhaps this is a time for humility.'
Arthur later tells his father that the assassin attacked him while he was hunting the beast, and that he was injured while killing him. Uther wishes to go to war with Odin, but Arthur urges his father to come to a peaceful resolution, saying it was grief for his son that motivated him. Arthur and Gwen exchange glances across the room, but later Arthur tells Gwen that his father would never understand what happened between them. Gwen tells Arthur he does not have to explain himself, and that perhaps, when he is King, things will be different.