But Gaius is too busy to leave Camelot - he's tending to citizens affected by a 'sweating sickness', apparently! He suggests sending Merlin in his place, and though Arthur scoffs, Gaius insists that his ward is up to the job!
Merlin rides to Longstead, accompanied by Gwen, Mary and the knights. Once he's left alone with those struck by the mystery sickness, Merlin tries to use his magic to cure them, but it has no effect - could their malady have been caused by sorcery?
That night, Merlin spots something moving outside of his lodgings. He nervously sneaks outside, but finds only Gwaine prowling around, summoned by a call of nature! The next day, the condition of the sick has worsened - they are on death's door...
Merlin, Gwen and the knights ride back towards Camelot, but en-route they spot a pack of bandits. When Gwaine spots them harassing a young woman, the chivalrous Knight leaps into action and a battle erupts between the knights and the bandits!
The bandits scatter and Percival comforts the shaken woman - her name is Lamia. The knights decide to take Lamia with them, and, as the sun sets, they prepare to make camp in the woods. But Lamia appears anxious around Merlin, and Percival's mood quickly shifts - he angrily warns the boy wizard to keep away from their new companion.
In the night, Percival hears Lamia weeping and hugs her tight, but as they embrace, Lamia's eyes transform and become snake-like! Back in Camelot, Arthur worries that Merlin, Gwen and his knights have not returned. Gaius is now available again, and Arthur tells the old physician that they must ride for Longstead at first light.
At Lamia's request, the smitten knights agree to escort her home. When Merlin reminds them that they need to fetch help for the people of Longstead, the knights angrily turn on him. Lamia's influence has affected them all, with only Merlin and Gwen remaining unaffected.
When Arthur's party rides out, they discover a string of fallen bodies - a band of slave-traders have been felled, but have not a scratch on them. Nearby, a carriage lies wrecked - something has escaped from within.
One trader is still alive and Arthur decides to take the injured man with them. Meanwhile, Lamia's influence over the knights grows, with a jealous Leon and Gwaine coming to blows. Arthur's party arrives in Longstead, but finds no sign of Merlin and the others. Gaius inspects the sick and confirms the King's worst fears: "There is sorcery at work here."
When the slave-trader recovers, he reveals that he and his men became enchanted by Lamia - fights and quarrels broke out amongst them. Turns out that Lamia is not the girl's name, but her race - a creature of magic! In the dark times of the old religion, the blood of a girl was mingled with that of a serpent - the result, Lamia! "They could transform at will," explains Gaius. "And become hideous monsters."
The Lamia is able to suck the life from a man with a single embrace, and that night, Lamia makes a move on Elyan. As they share a kiss, he collapses to the ground, weak and defenceless.
In the morning, the other Knights find Elyan's prone body. But they're still under Lamia's control and insist on following her to a nearby castle, despite Merlin and Gwen's protests. The pair realise that they cannot reason with the knights, so decide to leave behind strips of Gwen's tunic as a trail for Arthur...
On their way to the castle, the knights descend into a dark cave, but Lamia vanishes. They soon find themselves in a grim chamber littered with skeletons. Merlin tries to warn them that Lamia has led the group into a trap, but the knights violently reject his claims and Leon throws the young warlock to the ground.
Leon and Percival set off in search of Lamia and Gwaine hunts for wood to build a fire, while Elyan's condition worsens. Caring for her brother, Gwen wonders why Merlin has not fallen under Lamia's spell like the others (calm down, Merther fans!).
Merlin hears cries and when he dashes off to investigate, he finds a weakened Gwaine, struck by Lamia's touch! Meanwhile, back in the forest, Arthur discovers the material torn from Gwen's tunic and follows the trail...
In the cave, Percival follows the sound of Lamia's cries, but once he finds her, she plants a kiss on him and he too is struck with the illness. Sir Leon witnesses the attack, and, realising the truth, charges at Lamia, screaming in fury - but she quickly dispatches him with her magic!
Merlin arrives soon after to confront Lamia, and when she attacks, he uses his powers to spear her with Leon's sword. But Lamia survives - shedding her human form, she transforms into a monstrous, tentacled reptilian beast.
Merlin escapes, running back to Gwen, but Lamia is in hot pursuit and one of its tentacles snatch hold of his leg. Gwen rushes at Lamia, stabbing the creature, but it pushes forward and descends on her and Merlin...
Just as Merlin prepares to use his magic in front of Gwen, Arthur arrives, striking Lamia dead. "It's *almost* good to see you, Merlin!" he tells his manservant with a cheeky grin.
With the help of Gaius, the villagers of Longstead are cured of their affliction, along with the knights. Arthur can't resist winding Merlin up by pointing out that he had to be saved by a woman! But the King is really proud of Gwen and tells her as much, praising her courage under fire.
• Despite their addition as series regulars, the knights of Camelot have felt a little wasted so far this series, so it's great to see an episode that focuses on Leon, Gwaine, Percival and Elyan.
• While 'Lamia' is an entertaining episode, it's very much a standalone instalment, so any fans expecting progress on the 'traitor in Camelot' arc or another incident in Morgana's reign of terror will be disappointed.
• It's great to see Merlin showcasing his medical skills during to his trip to Longstead - these days he certainly feels less like a boy and more like the 'young man' that the opening narration credits him with being!
• It's also fun to see Merlin and Gwen paired off - Colin Morgan and Angel Coulby have great chemistry and, beyond series one, the two characters haven't spent nearly enough time together.