Princess Mithian arrives at Camelot in the dead of night, and the next morning, after Gaius has attended to her and her servant's exhaustion, she meets with Arthur and tells him that the army of King Odin - the man who slaughtered Uther - seized their castle, and now her father's life is at stake. Arthur vows to help, but Gwen can't help but wonder if he's joining the fray as much for revenge as for Mithian's justice. And Merlin notices something odd about Mithian and her serving woman, Hilda, as if there's a secret that Mithian desperately wants to speak, but cannot.
Arthur brushes aside these worries and makes for the realm where Mithian's father, King Rodon, is held prisoner. On the journey, Hilda collapses, and when Gaius examines her, he discovers that she has the vital signs of a much younger person, prompting Gaius to agree with Merlin that something is amiss. The following day, Mithian asks Merlin to fill her water skin, and when he does, he sees the name Morgana etched on a rock. As he runs to warn his king that Morgana has used a powerful spell to disguise herself as Hilda, the sorceress uses her magic to throw him against a tree, knocking him unconscious.
Arthur knows there's not much time for Rodon, so he presses forward, leaving Gaius and Gwaine to look after Merlin. Mithian soon leads them to the appointed spot, where Odin and Morgana lie ready for him. The Camelot Knights are no match for the ambush, and Arthur finds himself at sword's end.
But Merlin is revived by Gaius's magical ministrations and he and Gwaine rush to save Arthur. Merlin discovers Arthur in the tomb of Hodor's ancestors and sets the cave trembling, hopefully trapping Morgana inside. Odin and his men pursue Arthur and Merlin while Mithian and her father escape. A fierce duel ensues between Arthur and Odin, but when Arthur has Odin at his mercy, Merlin convinces the young king to make peace with Odin. Arthur convinces Odin that the loss of life must end, and as a mule through mud, Odin agrees.
On returning to Camelot, Mithian is pained by her betrayal, but Arthur forgives her trespass - he would have acted the same way if his own father were in peril.