Returning from a long day of work, Gwen is kidnapped by a pair of rough-looking mercenaries. Dumped on the floor of a dark room, she is confronted by Cenred, who says he has something she might like to see. Gwen's face falls as she realises who it is: Elyan, her brother. Cenred makes an evil offer: In exchange for Elyan's freedom, Cenred wants Arthur brought before him.
Arthur sends Merlin to Gwen's house to investigate her disappearance. The young warlock is concerned to find the place empty, but spots something on the floor – a scrap of cloth. He picks it up, sniffs it, and recoils...it is clearly laced with some kind of drug.
Gwen is returned to Camelot and tries to hide her fear, but it isn't long before she breaks down and tells Merlin everything. He convinces her to tell the prince. Arthur is sure that Elyan is being held in the Castle of Fyrien, and reassures Gwen that her brother will be rescued.
Morgana, who had been listening at the door, meets with Morgause, who is pleased everything is going as planned. When Arthur tries to rescue Elyan, they will be ready and waiting. Once Arthur is dead, Morgana can take her place on the throne.
Merlin prepares the three horses for their expedition, but is shocked to see Morgana arrive in full riding gear. His astonishment grows into panic when he learns she intends to accompany them on their rescue mission. The four travellers make their way toward the Castle of Fyrien. As night looms, they decide to make a camp. Desperate for some alone time with Gwen, Arthur sends Merlin off to help Morgana collect firewood.
Alone in the camp, Arthur and Gwen share emotional intimacy. Gwen is worried for Arthur's safety. He reassures her that everything will be fine. Their physical closeness, whilst trying to light a fire, brings them inches from a kiss...when Merlin returns with the firewood.
Morgause, meanwhile, is happy to have an update on the Prince's plan, and gives her sister an enchanted ring that she must place at the entrance to the tunnels. It will act as a beacon for Cenred and his men, ensuring certain doom for Arthur.
The gang reach the foot of the cliffs, the Castle of Fyrien towering high above them. Merlin knows they are heading into a trap, but his please to Arthur go ignored. As the party enters, Morgana hangs back and casts the enchanted ring to the floor, a beacon of doom for Arthur.
In the dark, cobwebbed tunnel, the party hear the sound of Cenred's men. They flee, but are greeted by mercenaries on both sides. They are dumped before a very happy Cenred. Arthur implores him to let the others go, but Cenred says that no friend of Camelot is innocent and has them all taken away.
Elyan can't believe Gwen came back for him. Shocked the Prince has risked so much, he realises how different his sister's life has become. Arthur and Merlin escape from their cell and hurry to Gwen, defeating the thug guarding her cell. Arthur wants to know where Morgana is, but Gwen doesn't know. Once again, Merlin has his suspicions, but is unable to let his friends in on the truth.
As they race to the entrance of the tunnels, Arthur tells Merlin to get Elyan and Gwen to safety. Arthur has to go back for Morgana. When he reaches Cenred's throne room, he sees Cenred with a sword to a seemingly terrified Morgana's throat. Arthur edges closer, but is stopped by Morgause who has crept up behind him. She conjures a huge tower of flames and hurtles it straight at the Prince.
Merlin arrives just in time and incants a spell causing the tower of flames to explode. When the smoke clears, Morgause lies in a heap on the floor; Cenred is next to her. Arthur asks Morgana if she is alright, but she is rooted to the spot, grief-stricken at the sight of her sister's apparent death.
Merlin makes his entrance. They are just about to leave when Cenred brings a sword crashing down on Arthur. The Prince barely parries the blow in time and tells Merlin to get Morgana out of there. The three escape, and are greeted by Gwen and Elyan,
Elyan has determined that he doesn't want to lose his sister again and intends to make a life for himself back in Camelot. Although dubious at first, Gwen warms to the idea and they all head back home. Later, Gwen visits Arthur in his chambers and thanks him for rescuing her brother. To his own amazement, he tells her that in no uncertain terms it's what you do when you love someone.
Merlin has pushed the limits of his strength to save Arthur, but even he may not be powerful enough to rescue Camelot€™s king.
In the battle foretold by the prophecies, all give their best, and Merlin wins the day.
Morgana may have one last trick up her sleeve to win the war.