Morgana has a nightmare in which Prince Arthur is drowned by a beautiful young woman. While out hunting with Merlin, Arthur comes across this woman and her elderly father being attacked by robbers. He fights the robbers off with the (unknowing) aid of Merlin's magic. The old man is brought before King Uther. He introduces himself as Aulfric and the girl as his daughter, Sophia. Aulfric is the heir to another kingdom ("TÃr-MÃ²r" - Gaelic for "Mainland") which was ransacked by raiders, and is travelling west with Sophia to join other family. Uther invites them to stay in Camelot for a while.
Morgana is horrified to see Sophia moving into the room next to Arthur's. She tells Gaius about her vision of Sophia and the fact that Sophia is now staying at the castle. Gaius tries to reassure her and gives her a sleeping draught, advising her not to worry Uther. Morgana is not convinced.
Aulfric is seen returning to the forest and using magic to kill the only one of the bandits who survived. He had paid them to pretend to attack him and Sophia. Morgana has another dream about Sophia drowning Arthur. The next morning Arthur goes riding with Sophia after making Merlin promise to cover for him, as he was supposed to be on patrol. Gwen (Guinevere) encourages Morgana to tell the king about her dream but Morgana fears Uther would punish her for use of sorcery. She resolves to stop Sophia herself. Meanwhile, Uther discovers that Arthur is missing. Merlin takes the blame for not giving Arthur the order to go on patrol, so Uther has him thrown into the stocks.
In the forest Sophia is about to use witchcraft against Arthur, but they are interrupted by the royal knights. He decides to go back to Camelot rather than taking her to the river. When Merlin returns home and explains why he has been in the stocks, Gaius is reminded of Morgana's dream. He examines Aulfric's possessions and finds his staff. The staff is carved with Ogham runes, which add to the mystery of just who Aulfric and Sophia are. He is then caught snooping by Aulfric, but manages to escape, but not before seeing Aulfric's eyes flash red with anger. Sophia is also angrily confronted by Morgana, who swears to hunt her down if she harms Arthur.
Aulfric wants to know why Sophia has not completed her mission. Their aim is to re-open and enter "the gates of Avalon". Aulfric warns her to hurry and they agree that their enchantment must be completed the following day. Morgana tries to warn Arthur about her dreams, but he does not believe her, arrogantly believing she is trying to confess her feelings for him (something Morgana strongly denies).
The next day Arthur prepares to go riding with Sophia again rather than attend the appointment of a new royal knight. Merlin is again thrown into the stocks. In the forest Sophia casts a spell which brings Arthur under her control. Merlin returns and Gaius tells him that Morgana is a Seer, born with the power to view the future in her dreams. He has known for a while about her ability but kept this quiet in order to protect her from Uther (he considers the ability to be too close to magic). He suspects that Arthur is indeed in danger. That evening Merlin overhears Sophia telling Aulfric that her magic has worked, and tomorrow they will be able to use Arthur for their own purposes. Aulfric says he will go to speak to "the elders."
Merlin follows Aulfric to the river, where he summons the Sidhe elders, who resemble fairies. It emerges that they were exiled from Avalon, the land of immortal life, because Aulfric killed a Sidhe. Aulfric must remain in a mortal body forever, but Sophia may return to Avalon again if they sacrifice a mortal prince to the Sidhe elders. Merlin tells Gaius everything.
Arthur requests an audience with the king and asks for permission to marry Sophia, which is denied. Uther believes that his son is too young, and threatens to have Aulfric and Sophia arrested if Arthur defies him. Morgana very nearly tells Uther about her dreams but cannot. Merlin attempts to warn Arthur but is caught by Aulfric and Sophia. Arthur's eyes magically glow red and Aulfric knocks out Merlin using magic. Morgana sees Arthur leave with Sophia and Aulfric, and runs to Gaius: he tells her to remain calm and leave the matter to him.
Gaius finds and tells a dazed Merlin that Sophia and Aulfric have taken Arthur away. Merlin rushes into the forest in time to see them beginning the ceremony to return Sophia to Avalon. She is told that if the elders do not get Arthur's soul, they will take hers instead. Merlin arrives as Sophia has magically suspended Arthur under the river. He magically grabs the staff she carries, which was lying on the bank, and uses a spell with the staff that kills both her and Aulfric. He wades into the river and saves Arthur.
Arthur awakens back at Camelot with no memory of what Sophia did. Merlin and Gaius tell him that he eloped with Sophia and Merlin knocked him out and brought him back to Camelot. Arthur is horrified that Merlin could overpower him and that he ever wanted to marry Sophia. He then forces Gaius and Merlin not to tell anyone what happened. Merlin again takes the blame so Uther has him placed in the stocks yet again. Gaius later gives Morgana another sleeping potion and makes Merlin swear never to let her know about her powers, or the possibility that she is a sorceress. She suffers another nightmare that night.