A mysterious swordsman breaks into the throne room and throws a gauntlet at Arthur's feet. The Prince accepts the challenge and orders the stranger to identify himself. The swordsman removes their helmet and we realize it is a beautiful woman, Morgause.
The battle is epic. Finally, Morgause stumbles. Arthur nobly stands back and allows her to retrieve her weapon. The fight resumes, and Morgause prevails. Arthur is on the ground, a sword at his throat. Merlin is about to intervene with magic when Morgause makes an offer - if Arthur promises to come to her and accept the challenge she sets him, she will spare his life. Arthur agrees. Morgana is drawn to Morgause's chambers, where the stranger expresses concern for Morgana. When Morgana admits she has trouble sleeping, Morgause offers her a healing bracelet as a gift. That night, as Morgana sleeps fitfully, Morgause appears in her room. She incants a spell over the bracelet before leaving it at Morgana's bedside.
Arthur prepares to go after Morgause, but Uther refuses to let him leave, confining Arthur to his chambers. Arthur refuses to tow the line and gets Merlin to help him break out of the castle. Together, they set off to find Morgause. Part way through the journey, Arthur is forced to admit he doesn't know where he is going. Merlin is understandably nervous, but Arthur is determined to press on.
Merlin and Arthur reach a lake where the path stops. Merlin watches as Arthur's horse wades into the pool and straight through a cascading waterfall. Merlin and Arthur arrive on the other side of the waterfall, into the shadow of a ruined castle. Merlin is worried - there can be no doubt that Morgause is an enchantress. Arthur agrees, but says he has to see this through to the end.
In the ruined castle, Morgause appears and congratulates Arthur on staying true to his word. She then reveals the challenge, telling Arthur to place his head on a chopping block. Merlin's appalled when Arthur does as he is bid. Morgause raises an axe...but she does not bring it down. Instead, she praises Arthur's bravery. He has shown himself worthy, so she will grant him one wish. Arthur wants to hear about his mother. Morgause says she can go one better; Arthur can see Ygraine if he wants to.
A worried Gaius goes to visit Uther. The physician informs his King that Morgause had a bracelet which bore the mark of the house of Gorlois. Morgause is Morgana's half-sister, whom all believed had died at birth. Gaius admits he smuggled the child out of Camelot and entrusted her to the high priestesses of the Old Religion. He swore a solemn oath never to tell a soul and he is only breaking his word now because he fears for Arthur.
Morgause uses powerful magic to bring Ygraine back from the dead. A tearful Arthur is united with his mother for the first time. He finally learns the devastating truth about his own birth: How Uther's obsession with having a son made him turn to magic, and how Ygraine's life was taken in exchange. Arthur is devastated and angry. Morgause watches with a satisfied smile.
Arthur returns to Camelot, enters the citadel and runs to challenge his father. Uther vehemently denies the claims, but Arthur throws down his gauntlet, telling Uther he has Ygraine's blood on his hands. Uther refuses to fight, but Arthur will not take no for an answer.
Merlin confronts Gaius about Ygraine's story. Gaius cannot lie and admits that it's true, though he presents a strong case for Uther. Arthur, meanwhile, has goaded Uther into a fight, determined to get revenge, and soon has his sword to the King's throat. But Merlin appeals to his friend. He knows there is only one way to stop this fight, so he lies. He tells Arthur that Morgause's words were false and his mother was merely an illusion. Morgause has magic and therefore is not to be trusted. Moved, Arthur challenges Uther to swear that he was not responsible for his mother's death. Uther swears on his life - he loved Ygraine and would never have hurt her. Arthur drops his sword and Uther gathers his son into his arms.
Much to Merlin's surprise, Uther comes to thank him, but leaves with a final message; if Merlin ever tells a soul about what he witnessed today, he will lose his head. Meanwhile, Morgause looks into a shard of crystal and utters a spell. She sees the image of father and son at peace. She is furious.
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