4.13 EPISODE SUMMARY! MAJOR SPOILERS

All hope is lost for Camelot. King Arthur has been betrayed by his uncle, Agravaine. Morgana and Helios have captured the citadel. Merlin and Arthur, along with Gwen, Gaius, the Knights of the Round Table, Tristan and Isolde have taken refuge in Merlin’s village of Ealdor. Arthur feels all hope is lost, the army of Morgana and Helios too mighty. Merlin decides to create a story of a great sword stuck in a stone that only the true King can wield. Arthur pulls the sword, Excalibur, and his confidence is restored. And it could not have come at a better time. Morgana’s men are marching on Ealdor. Merlin, Arthur, Gwen, Tristan and Isolde decide to use this as a chance to sneak into the city and destroy Morgana once and for all. With Hunith and the villagers ready to fight by their side, the Knights do battle with the overwhelming force. When defeat is near, they are saved by the Great Dragon and Aithusa, who has grown a bit since he was first seen.

Since Tristan and Isolde are unknown to the soldiers of Helios, they are able to bypass the guards in the lower town. They sneak into the citadel, making way for the others. Arthur, along with Tristan and Isolde find Helios in the council chambers. Merlin and Gwen sneak into the dungeon to free the remaining Knights of Camelot. Agravaine, who along with Helios has secretly been plotting to turn on Morgana, makes his move as she goes to the council chambers. He succeeds in driving a blade through her back, but she manages to drive one of her own through his chest as he stands over her gloating. Arthur fights and kills Helios. Merlin and Gwen find Morgana near death in the hall. Arthur and the others join them. With her dying breath, Morgana tells them that she only ever wanted to live in peace, but Uther and Arthur would not let her and others like her do so and she was forced to do what she’s done.

Arthur and Gwen hold a conversation, deciding that once they are married and Camelot united again, the old laws must be re-written to ensure change. As part of this change, magic will no longer be banned. Their wedding, the coronation of Queen Gwen, takes place. That evening, Morgana wakes up in a strange place to her: Avalon. The new elders tell her that her time is not yet come. Though she has fulfilled her destiny, to bring magic back to the land, now she must start a new journey: Becoming Queen of Avalon. To do this, she must seek out those she wronged and find their forgiveness, though she will be tested. Not a moment later she wakes up beside a lake, a familiar voice calling out to her. It is the druid Mordred, grownup and quite handsome. After explaining where he has been since they last met, he tells Morgana that he has heard she is searching for Emrys. Morgana is enraged to discover that Merlin is Emrys. Back in Camelot, Arthur enters his chambers, hoping to find Gwen but instead finding Merlin. Merlin recites a spell and a large flame appears in his hand. Arthur cannot believe what he has seen: Merlin, his manservant and friend, is a sorcerer.             

FROM: http://www.fanpop.com/spots/merlin-on-bbc/forum/post/158401/title/major-4x13-spoilers