Arthur finally submits to the reality that Gwen was profoundly changed by magic while she was in the Dark Tower in Morgana's captivity, and Gaius manages to convince him that the only way to restore her to her true self is through sorcery. Arthur despises the idea, but Gwen's treachery runs so deep that Arthur has no other option. The solution lies beyond Gaius's knowledge, so Merlin must seek the counsel of a tricky earth spirit call the Dochraid.
Since Gaius has warned him not to trust the crone, Merlin visits her in disguise, but she quickly knows that he is Emrys, there to save Queen Guinevere, and as the Dochraid is allied with Morgana, she refuses to help. Merlin cuts her arm with a dragonfire-forged blade and forces from her the information that the only way to lift Morgana's spell is to bring Gwen to a lake called the Cauldron of Arianrhod and summon the White Goddess. Gwen must enter the cauldron of her own accord, for if she is tricked or forced, the dark magic will not be cleansed. Merlin doesn't like the idea of Arthur going along because it means that he won't be able to perform magic, but Gaius knows that Arthur is the only one that can still reach the true Gwen and convince her to enter the cauldron. He therefore glibly arranges for Merlin to summon the goddess in the guise of an old woman.
The cauldron is a three-day journey from Camelot, meaning that just to get Gwen there will involve dosing her with a large amount of belladonna. After slipping the tincture into her wine, Merlin and Gaius wheel her out of the castle in a barrow, awkwardly explaining their lumpy, covered parcel to Gwaine and Mordred, and hoping they didn't see Gwen's hand pop out from under the sheet as they left. Merlin meets Arthur outside the castle with the sleeping Gwen and they begin the journey.
As they wend their way along a sheer mountain pass, Merlin gets the sense that someone's watching them, and moments later, he slips down the cliff face and is knocked unconscious. Arthur tries to save him, but the loose rocks tumble and trap him by the arm. All the travelers would have perished if Mordred - the presence Merlin perceived earlier - hadn't appeared. Merlin still doesn't trust Mordred, but he keeps his opinion safe from Arthur.
The following day finds them walking distance from the cauldron, but the Dochraid has sent word to Morgana of Arthur's plans, and she and Aithusa attack them. Merlin urges Arthur to take Gwen and keep going and, when they are out of sight, he commands Aithusa to retreat. Mordred is captured by Morgana, who demands he tell her where Emrys is, but Mordred is unaware he travels in the presence of such a power. He manages to knock Morgana off her feet and make an escape. Merlin slinks off to don his absurd disguise, but Mordred notices that Merlin is missing, to which the "sorceress" replies that she is holding the boy as surety.
When Gwen awakes, she's furious that she's been brought to this strange place, and pelts Arthur with oaths of loyalty to Morgana. The "sorceress" impresses on Arthur the need to help Gwen remember her true self, so he reminds her of her answer to his proposal of marriage: "With all my heart." Repeating these four words, he walks slowly into the cauldron and beckons Gwen to follow him. Fighting against powerful magic, Gwen does so, Merlin summons the White Goddess, and Gwen is restored. As a token of gratitude, the sorceress requests that Arthur remember what saved his queen, and reminds him that evil is not born of sorcery, but of the heart of a few who practice it.
When they have returned to the castle, Mordred tells Merlin that he knows it was Merlin, himself, who performed that spell, and tells him the secret is safe, imploring once more that Merlin believe in his good intentions. But Merlin cannot ignore the warning signs, and will not yield his trust to the young knight.