A paranoid Uther summons to Camelot the feared and revered witchfinder, Aredian. In the days of the Great Purge, Aredian exposed countless witches and warlocks with his grim interrogation techniques, helping Uther drive sorcery underground. Gaius tries to impress upon Merlin how serious the situation is, but Morgana needs no convincing. She's terrified Aredian will see straight through her.
Merlin is first to be interrogated, and while he's confident he didn't incriminate himself, Gaius isn't sure. Further witnesses come forward with stories of magic around Camelot. Merlin knows he wasn't responsible, but Aredian makes his declaration to Uther - Merlin is the sorcerer!
Gaius' chambers are overturned by the guards, and Aredian discovers a magical amulet. Gaius speaks up, admitting the amulet belongs to him. Merlin is released, and Gaius is jailed and interrogated in his place. Uther surprises Aredian by questioning his judgement, insisting Gaius is a trusted ally in the fight against magic. But Aredian reminds Uther that Gaius was once a sorcerer. Though reluctant, Uther relents. As Aredian deprives Gaius of food, sleep and water, Merlin and Gwen appeal to Arthur for help. He insists there's nothing he can do, and implies that Merlin might be in cahoots with Gaius, and comes down hard on Gwen when she dares to challenge him. But it is clear Arthur's friends have rattled him.
Merlin asks the Dragon for help, but to no avail; the Dragon won't encourage anything that might endanger Merlin's life. Merlin is determined to confess to save his mentor, but the Dragon makes it clear that if Merlin dies at Aredian's hand, then the lives of all those Merlin is destined to protect in the future will be doomed.
Gaius is bowed but not broken. He does, however, accidentally let slip that Morgana has bad dreams. Aredian convinces Uther to allow him to ask his ward a few "innocent" questions, warning that she may have been corrupted by Gaius. Morgana manages to hold it together under questioning, but Aredian gets from her more ammo to use against Gaius..
Trapped, Gaius confesses to Uther. Horrified, Merlin attacks Aredian in front of the King, and Arthur drags him off to the cells. Merlin tries to fight Arthur as well, but Arthur reveals his true colours by sneaking Merlin in to see Gaius. Gaius makes Merlin promise to let him go quietly, but when Merlin hears that the amulet wasn't Gaius' at all, there's no way Merlin will agree to let this go. The amulet isn't Merlin's either, so that means Aredian set them up!
As Morgana cracks and confesses to Aredian, Merlin and Gwen scramble to prove that the witchfinder is corrupt. Their quick-thinking detective work uncovers how Aredian used hallucinogens to provoke the false eye-witness reports of magic use. But they can't help Gaius without definitive proof, so Merlin risks his neck and sneaks into Aredian's chambers to plant some "evidence" of his own.
Gaius is seconds away from the flames and it's left to Gwen to stall his execution. She pleads with Arthur directly, once more challenging him to do the right thing and prove himself the good and fair leader she knows him to be. It works. Arthur intervenes and Merlin is allowed to present his arguments to Uther. Aredian's chamber is searched and the damning evidence is found. Aredian is revealed to be not only corrupt but also, thanks to Merlin's inventiveness, a sorcerer as well! Aredian takes Morgana hostage, but it's too late; Merlin incants a spell that sends Aredian out a window to his death.
Gaius accepts Uther's apology, but the physician won't let his King off lightly. He reprimands Uther for allowing his hatred of magic to twist his thinking, and a chastened Uther acknowledges his failing.
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