Knight Valiant, a renowned warrior, has asked a wizard to make him a beautiful shield decorated with painted snakes. The wizard uses a spell to bring the snakes to life, and is then killed by the snakes at Valiant's command. Valiant rides to Camelot and enters the annual sword tournament, which is traditionally won by Prince Arthur.
Meanwhile, Arthur and Merlin rehearse swordfighting. At home with Gaius, Merlin uses magic to summon a scroll that he must learn before the tournament tomorrow. Gaius warns him not to be caught using magic. When Merlin complains about having to work as Arthur's servant, Gaius states that Arthur is under a lot of pressure from his role as future king.
King Uther opens the tournament and both Arthur and Valiant easily defeat their first few opponents. Merlin later uses magic to complete his long list of chores for Arthur and instead uses the time to study from the book of sorcery that Gaius gave him. Back at the castle, Valiant is introduced to the king's ward Morgana and flirts with her. He vows to win the tournament so that he can have the privilege of escorting her to the celebratory feast.
While replacing Arthur's armour the following morning, Merlin hears the sound of snakes hissing. He examines the snakes on Valiant's shield and briefly sees one of them winking at him. Valiant comes in and confronts him, but Merlin successfully pretends not to have noticed anything. Arthur is impressed at Merlin's work on his armour, while Merlin privately admits to enjoying his job at the castle. During the day's fighting Valiant commands his snakes to attack an opponent. The injured knight, Ewan, is brought to Gaius and treated for the snake bite. Merlin rapidly works out what has been going on. He spies on Valiant and sees him feeding live mice to the snakes. Gaius cautions Merlin not to tell the king until it can be proven that Valiant has used magic to cheat.
Arthur and Valiant become the two competitors who will appear in the final. Gaius agrees with Merlin that Valiant cannot be allowed to kill Arthur. They conclude that if they can get the right antidote, Ewan will recover and can personally tell the king about the snakes. While Valiant is at dinner, Merlin goes into his chamber and examines the shield. He decapitates one of the snakes and saves its head. Valiant notices him running away and quickly discovers that Merlin has found out his secret. Meanwhile, Morgana has a dream in which Arthur is killed by Valiant.
Merlin presents the snake's head to Arthur and tells him what Valiant has done. He swears he is telling the truth, and Arthur requests an audience with the king. Ewan awakes but is again bitten by one of the snakes, this time fatally. When Ewan is found to be dead, Uther refuses to listen. He furiously chastises Arthur and has Merlin seized by the guards. Valiant requests for Merlin not to be punished and asks Arthur to concede the tournament to him. Arthur refuses, but apologizes to Valiant and withdraws his accusations.
A devastated Arthur orders Merlin to leave the castle. Merlin visits the Great Dragon and is again told that his destiny lies with Arthur. Gwen encourages him not to give up, and Merlin has an idea. Using a statue and Gaius's magic book, he attempts the spell that will bring the snakes to life. Arthur knows that Merlin is telling the truth about Valiant but still refuses to concede the tournament. He is willing to die rather than be thought a coward by his future subjects.
The morning of the final arrives. Both Arthur and Valiant are equally matched. As Arthur and Valiant struggle, Merlin uses the spell. The snakes are released from the shield and Valiant orders them to kill Arthur. The prince decapitates them with a sword thrown to him by Morgana, before fatally wounding Valiant. The crowd wildly applauds his bravery.
At the feast, Morgana teases Arthur about being rescued by a woman. He arrogantly declares that he needed no help but indirectly apologizes to Merlin, and reinstates Merlin as his servant.