In a dark cave, a troll downs a potion and lets out an earth-shattering scream of pain. Her man looks under the troll's hood and admires his work. Mistress and servant make their way towards Camelot. The troll pulls back her hood and smiles - she has been transformed into a beautiful woman.
The troll's servant approaches Merlin and Gaius and asks to see the King. Gaius rebuffs him until the man produces the seal of Tregor - the Pendragons have long been allied with the House of Tregor. Uther is amazed by the transformed troll, who calls herself Catrina. She "explains" she and her servant Jonas escaped the slaughter of House Tregor and hope they might stop for a night's rest. Uther insists.
Later, alone with Jonas, Catrina speaks openly - stage one of the plan is complete. Merlin delivers one of Gaius' potions to Catrina, narrowly missing the sight of her eating rotten fruit. Gaius told Merlin that Catrina would know what the potion was for but she disagrees and politely but firmly sends Merlin away.
Disturbed to hear this, Gaius fears the Lady is not who she says he is. When Catrina was a child Gaius treated her for an incurable bone disease and yet this Catrina suffers no pain and refused the one thing that might bring her some relief.
Gaius tells Uther about the matter of Catrina's disease. Uther reacts with irritation, but the veiled warning has obviously given him pause, because he later questions her on the topic. Catrina lies that her recovery was an unexpected miracle. She ramps up her seduction of Uther and goes in for a kiss. Though tempted, Uther pulls back from the embrace, citing Arthur's late mother as a reason to take things slowly.
Merlin trails Catrina deep down into the bowels of the castle. Finally, he comes to a foul-smelling chamber littered with troll droppings. In the nest lies Catrina, fast asleep and snoring. Shocked, Merlin runs off again. But not before Jonas sees him.
Catrina and Jonas use powerful dark magic to enchant a pendant. Later that evening, she gives Uther the enchanted pendant as a "departing gift". The pendant glows gently around the King's neck as Catrina tells him that she is his heart's desire and no one else's opinion matters. Dreamy-eyed, Uther agrees.
The next day, Uther happily tells Gaius that Catrina will be extending her stay in Camelot indefinitely. Alarmed, Gaius warns Merlin that the only way forward is to expose Catrina to the King in her true form. But Uther makes an announcement to the court - he and Catrina are to marry! Everyone but Gaius and Merlin bursts into rapturous applause. Gaius flashes a look to Merlin - he must act quickly. Merlin begins to mutter a spell from the back of the court. Catrina shifts uncomfortably, but fights the magic.
Later, Jonas lets Merlin finds him crying and begs for help escaping Catrina, who he claims is controlling him. Merlin's wary, but then Jonas offers to show Merlin where Catrina stores the potion that keeps her in human form. Merlin realises he must take the risk, and follows Jonas down into Camelot's tunnels, where he comes face to face with Catrina. She laughs and unleashes a bolt of magic that brings the ceiling down, burying him.
As preparations for the wedding get underway, the young sorcerer comes to, trapped in an underground prison. As the wedding guests arrive far above him, Merlin is utterly spent. But with the future of Camelot at stake he raises his weary hands one last time and incants a spell. CRASH! The wall of rocks tumbles down.
Uther enters the throne room and takes his place, ready to receive Catrina. The ceremony begins as Merlin skids around a corner...and gets stopped by Jonas!
Jonas and Merlin face off. Merlin makes to charge past, but WHAM! Jonas hurls Merlin back with superhuman force, then lifts him into the air. Merlin finds the strength to kick out - he flies one way and Jonas the other. They both scramble back to their feet, but Merlin is quicker this time. He launches a shockwave that sends Jonas flying, knocking him out.
Merlin catches his breath and bursts into the throne room...just in time to see Uther and Catrina made man and wife. Uther kisses his bride. Camelot has a new queen.
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