Morgana and Odin have invaded the kingdom of Nemeth, friendly to Camelot and home to King Rodor and Princess Mithian. They’ve sacked the castle and taken it over, taking the royal family prisoners.
Morgana strikes a bargain with Odin. If he can do the same to Camelot, he can have Arthur.
Back in Camelot, a wounded Princess Mithian arrives, with Hilda, her elderly servant, in something of a state. She’s taken to Gaius, who prescribes bed-rest. Arthur is worried that something awful has happened but is prevented from asking by Merlin, who says she needs rest.
The old servant, having been left with Mithian, and ensured that the room is clear, suddenly bends over in pain. A transformation is taking place. WHOAH! SHE’S MORGANA! She’s holding Rodor hostage and forcing Mithian to convince Arthur to go on a rescue mission to the forests of Gedref, which she does.
Trouble is, no one knows Morgana is there, she’s gone back into her old woman disguise. Arthur plans a surprise raid, banking on Odin’s lack of knowledge to secure Rodor’s safe return. But of course it’s all a trick, and anyway Morgana is blabbing information to Odin, using a magic carrier crow. Then she uses her magic to duff Mithian up. There will be an ambush, and things look bleak.
A chance encounter with Merlin leaves the young wizard looking troubled. He sees fear in Mithian’s eyes, and Hilda seems odd, somehow. Not like herself.
On the other side of the castle, Gwen accuses Arthur of hatching a revenge attack on Odin, who, let’s not forget, killed his father (coincidentally Arthur killed Odin’s son). Arthur’s having none of it, but this doesn’t seem to reassure her. And then Mithian manages to sneak out of her chambers, and rushes to find help. But Hilda finds her, and uses a magical bangle to hurt her. Gwen sees the pair, but is reassured that all is well, even though it is not.
And off Arthur goes to Gedref, despite Merlin’s misgivings. And he’s taken Mithian with him. Merlin spots a burn on Mithian’s from the magic bangle, which only increases his sense of foreboding. He tries to talk it over with Arthur, who simply asks what else they can do but let events unfold as they are. Merlin is at a loss to reply.
Then, a lucky break. Hilda falls, exhausted, and has to receive treatment from Gaius. He spots that she’s a lot younger on the inside than on the outside, but it still isn’t enough for Merlin to do anything about. The next morning, Mithian warns Merlin by writing Morgana’s name on a stone by the stream in which he is filling water bottles. But Morgana attacks, magically throttling Merlin and throwing him into a tree so that he he blacks out.
Arthur decides to push ahead, leaving Gwaine and Gaius with Merlin, and taking Hilda and Mithian with him. But Merlin appears resistant to waking up, until Gaius hits him with some magic smoke. He wakes, and raises the alarm, but there’s only Gaius there to hear it.
Gawain returns, and they rush off after Arthur, who has been lead by Mithin to some caves, where she says her father is hiding. As they enter, Merlin and Gwaine spot a platoon of Odin’s men heading towards the same spot. They enter the caves and overpower the men of Camelot, despite a spirited defence from Arthur and Percival.
Morgana reveals herself, Odin enters. It’s time for Arthur to die. Unless Merlin and Gwaine can do something about it. First they sneak past the guards, then Merlin causes an earthquake, just as Odin’s sword is heading for Arthur’s neck. Percival and Arthur use the confusion to escape, with Merlin pausing only to biff Morgana with a biffing spell.
They rescue Rodor and Mithian, and run into the forest, chased by Odin and his men. Arthur and Merlin, attempting to lead Odin away, find themselves in a dead end. Arthur and Odin fight, and Arthur wins.
Merlin counsels Arthur not to kill Odin, or risk failing to unite the land. Arthur proposes a truce, a restored throne of Nemeth and a kind of united kingdom (no pun intended). Odin agrees, eventually, and everyone goes home.
Back in Camelot, Mithian tries to apologize, but Arthur won’t hear of it, and even risks a compliment for Merlin as a result. However, Merlin is concerned that Morgana’s power is growing, and that she will one day surpass him, leaving Arthur, and therefore Camelot and the kingdoms, un-protected.