A mysterious illness draws Merlin to a distant village. But it soon becomes clear that there is more at stake than just his patients' well-being as something sinister turns the air sour. Realizing he is out of his depth, he resolves to return to Camelot and fetch Gaius.
On the way, however, Merlin and the knights encounter a girl being accosted by bandits. They rescue her, but all is not as it seems, as time spent with the girl begins to turn the knights hostile and make returning her home a fixation in their minds.
Fearing for Merlin and the lost knights, Arthur, Gaius and Agravaine visit the original village. There, Gaius concludes that the cause of the illness was Lamia, who is not a girl, as she appears, but actually a magical creature in disguise. Arthur and Agravaine go in search of their lost party and, despite the latter's efforts to foil the effort, eventually track them to an abandoned castle with some help from Gwen.
Meanwhile, at the castle, the Lamia is picking off the knights one by one. Merlin remains immune, however; a happenstance of which Gwen takes note. Merlin eventually engages in a magical duel with the creature, which has now transformed into a monstrous creature, but just as Merlin is about to unleash his strongest attack, Arthur arrives, and slays the beast with a spear. The party then returns to Camelot, where Gaius is able to heal those afflicted with the Lamia's infectious curse.
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