The new queen of Camelot proves to be far crueler than Uther, who is made to watch from his dungeon as she slaughters innocents. Guinevere helps Sir Leon escape from prison and joins her brother Elyan, Gaius, Merlin, Arthur and Gwaine, who are hiding in the woods. Soldiers follow but they are saved by Lancelot and a youth of immense strength, Percival. Merlin looks into the waters of Avalon in the phial given him by the Fisher King, which directs him to Excalibur, the sword which will defeat the enemy. The fugitives find a round table, around which they movingly declare allegiance to each other, and Arthur knights all the young men - except, of course, Merlin, though Lancelot, aware of his powers, tells Merlin he is the bravest of them all. The group launches its invasion of Camelot, which gives Gaius his finest hour as the sisters are routed, largely due to Merlin's use of Excalibur. However, he places the sword in a rock until it can be used again.
A week into the reign of deposed Uther's bastard daughter Morgana and her even more evil sorceress sister Morgause, they attempt to impose their authority by violence, executing loyal knights. Arthur, hiding with Merlin, Gaius and Gwaine, are soon joined by Lancelot, Percival, Gwen and her brother Elyan. The key to liberating Uther and all Camelot is overcoming the army of the undead, which loses its immortality only if the Cup of Life is emptied. Merlin summons the dragon to help him retrieve the dragon breath-forged sword Excalibur, the only weapon which slays the undead. His ultimate asset is the gift from the Fisher King, but ignores how to use it.
- Queen Morgana begins a reign of terrible evil over the innocent citizens of Camelot. With Morgause's immortal army at her command, it seems there is no hope for Uther, who must watch his Kingdom crumble under the unimaginable cruelty of his own daughter.
Meanwhile, on the outskirts of Camelot, Arthur regains his strength and, with Gwaine, Lancelot, Elyan and Percival at his side, plans a brave counter-attack. But Merlin knows that even Arthur cannot vanquish an immortal foe. Can he recover the great sword Excalibur from the Lake of Avalon in time to save Arthur from certain doom?