SEnchanted Uther couldn't be happier with his new Queen, but Catrina is not content with a title - she has a plan for absolute wealth and power. As the new King and Queen retire to their chambers after the wedding, Uther tries to get amorous, but Catrina feigns upset and tells Uther that Arthur's servant has stolen her precious family seal. Uther is furious and orders Merlin's immediate arrest.
Arthur warns Merlin to run. He reports to Uther that Merlin has escaped and, the following morning, the King's soldiers find clues that indicate he has fled for the Northern borders. Catrina is pleased. Soon, Camelot will be hers!
But Merlin is hiding in a secret space in Gaius' chambers. Gaius is clearly nervous and warns Merlin that he's playing a dangerous game, but Merlin knows he has no choice - he has to find the evidence to expose Catrina. He sneaks into the bowels of the castle and finally, amongst the filth, he finds a stash of empty bottles.
Gaius examines the liquid carefully. Perhaps Catrina does need a potion to transform. Together, Gaius and Merlin realise that, if Gaius can make a harmless liquid that looks and smells like the potion, Merlin can swap it for the real thing, and Uther will see Catrina as a troll. Gaius manages to make a harmless duplicate of the potion. Merlin dashes off to switch the bottles, narrowly avoiding being caught by Jonas. He watches from a secret hiding place as Catrina drinks the placebo potion. But is he too late?
Catrina's influence has clearly had an effect as Uther revokes Arthur's title of Crown Prince. Arthur is devastated, but Merlin is stricken for a very different reason: with Catrina the new heir, if Uther dies, a troll will inherit the throne!
At the lavish coronation ceremony, Gaius watches Catrina carefully. Sure enough, she begins to itch and strain - the potion is wearing off. But Catrina is too powerful. She manages to retain her human form long enough and the moment the papers are signed she flees from the gaze of the court.
But a concerned Arthur, Morgana and Uther dash after her. Merlin uses magic to stop Catrina evading them and, with a hideous cry she transforms into her full troll form. Arthur and Morgana are stunned, but Uther doesn't bat an eyelid - he's too enchanted to see the truth.
Merlin goes to the Great Dragon for help. At first, the beast refuses, but Merlin presses and the Dragon eventually admits that the there is one way to make the King see the truth. He must cry tears of pure remorse.
Merlin is forlorn, fearing Catrina has won. But Gaius knows that there is one thing Uther cares about more than any other in the world - Arthur. They must fake the Prince's death. They can slow down his pulse with a potion of rare herbs... but it's dangerous. Merlin must administer the antidote as soon as Uther's enchantment is broken or the Prince could really die.
Gaius announces Arthur's death, and a shocked Uther rushes to see his son's body. Merlin tries to stay close with the antidote but Jonas spots him and uses magic to knock him out. The bottle flies from his hand and shatters, and the precious liquid spills across the floor.
Uther is shocked by the sight of Arthur's dead body, and blames himself for Arthur's death. Overwhelmed with remorse, he begins to cry...he turns to Catrina for comfort and, for the first time, sees her true form. Catrina tries to enchant Uther again but it does not work - the spell has been broken.
Uther draws his sword and calls in the guard. But Catrina is strong and powerful and Uther watches in horror as she easily dispatches the guards before turning on him...
Merlin rushes to Arthur's side, praying he is not too late. The young warlock desperately wrings out his scarf (which he has used to mop up the antidote) over Arthur's mouth. One single drop of antidote falls on his lips, and Arthur begins to breathe again.
Catrina taunts Uther, then advances just as Arthur gets to his feet. Together, Arthur and Uther fight the troll Queen. Merlin uses magic to help Arthur and, eventually, the young Prince triumphs. Jonas and Catrina are killed. Uther is devastated that he married a Troll and furiously renews his war on magic. Arthur thanks Merlin for his bravery and help in defeating Catrina. For once, Merlin gets some of the recognition he deserves...
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