On their wayback to Camelot from their yearly sojourn to their father's grave, Gwen and Sir Elyan, accompanied by several knights, come upon a nest of snakes. The knights are thrown from their horses, and Elyan shouts to his sister to ride for the trees and make her escape. Elyan takes the bitten knights, Leon and Percival, back to Camelot, but when Arthur discovers Gwen is missing, the Round Table must ride again to recover her. Leon and Percival are recovered, but Merlin and Gaius note that a snake bite that severe should have killed them. The fact that they live is a sign of magic, and they suspect Morgana.
Indeed, after dragging the queen on foot behind her horse, Morgana locks Gwen in a dark chamber with eerie screeches and foul smells. Any kindness Morgana shows, Gwen spurns, but as visions of her brother, her king, and Merlin appear and taunt her, she becomes less able to refuse the friendship of her once foe.
After a long day's ride, Arthur and his company make camp, and Leon and Percival are shocked from slumber by a singular dream of a black spire of rock in the middle of a vast desert. Arthur recognizes this image as the Dark Tower, a tale told to young knights as the strongest caution - no one is known to have come out of the tower alive. Merlin tells Arthur that he believes this vision was placed in their heads by Morgana, who is leading them into a trap. Arthur doesn't doubt this, but since Gwen is in peril, they must forge on.
The only way there is through the Impenetrable Forest, which confounds their sense of direction at every turn. At night, Merlin gets a visit from Queen Mab, the fairy ruler of all lands of anguish. She calls Merlin, Emrys, and tells him he must trust his mind's eye. Further, she says, the Dark Tower is not a physical place, but the heart of fear in each of us. She prophesies that the tower will claim the life of one of his cohort, but before Merlin can ask who will die, Mab is gone.
The following day, Merlin finds the true path through the forest, and the knights reach the tower. Sir Elyan, wracked with guilt at having let his sister be captured by Morgana, forges ahead of the party and finds Gwen alone in a simple chamber. Gwen tries to warn Elyan to leave, for she's guarded by a sword that is enchanted to fight to the death, but Elyan won't abandon his sister. He tricks the sword to fly out of a window, but before it does, it strikes a fatal blow, and Elyan dies in his sister's arms.
The party returns to Camelot, grateful for Gwen's return but mournful of Elyan's loss. But that night, as Gwen lies sleeping in Arthur's arms, she wakes and slips into the forest to meet Morgana. The mandrake roots in Gwen's prison chamber worked a powerful magic, and she now believes Morgana is the only person she can trust - all friends at Camelot are now deadly foes.