Merlin relays his suspicions to Arthur and they shadow Guinevere as she goes to meet Morgana. Merlin learns from ancient priestess the Dochraid that the spell on the queen can be broken if she willingly enters the pool known as the Cauldron of Arianrhod whilst a sorcerer makes an incantation. Thus Arthur drugs his wife and, along with Merlin, takes her to the Cauldron, followed by a suspicious Mordred, who, unbeknown to the others, uses his own magical powers to thwart Morgana. At the Cauldron, Merlin vanishes and reappears (thanks to a sex-changing spell) as an old sorceress, the Dolna. Arthur gets Guinevere to recall their old love and join him in the pool of her own volition where Merlin can finally lift the curse with an incantation. In character as the Dolna, Merlin reminds the grateful Arthur that, as a reward, he should recognize that magic can be used for good. On the return journey to Camelot, Mordred implies to Merlin that, unlike the blinkered Arthur, he knew the true identity of the Dolna.
—don @ minifie-1