Atop a mountain, the troll Catrina takes a potion from her servant Jonas, which provides her with human form; part of a scheme to bring about the downfall of Camelot.
Merlin complains to Gaius about their usual boring routine. Jonas approachs them, asking them the whereabouts of Uther Pendragon, stating he has urgent business with the King. He hands Merlin the seal of the House of Tregor as Gaius questions Jonas as to how he got it. Jonas states it doesn't belong to him but to 'Lady Catrina', to whom both Merlin and Gaius immediately bow. Lady Catrina and Jonas are welcomed by Uther, under the premise that Catrina is the only survivor of the destruction of her city. Uther offers his help, and Catrina asks only for a bed for the night. Merlin comments on Catrina's bravery to Gaius, who seems to be bothered.
After being shown to her room, Catrina's reverts back to her troll form and exhibits her natural disgusting behaviour. Uther explains at dinner that the houses of Pendragon and Tregor have been in alliance with each other for some time, and as he lavishes compliments upon Catrina, Morgana exhibits amused expressions whilst Arthur looks quite disgusted and bored.
Merlin mentions that Catrina is of a different character as she doesn't treat him like dirt. Gaius comments that he had met Catrina before, and hands Merlin a potion to give to her. Catrina comments on Uther's stupidity and blindness by 'pretty things' as she eats rotten food, only to be interrupted by Merlin. He presents Catrina with Gaius' potion, only to be rebuffed. A confused Merlin leaves and confronts a bemused Gaius. Gaius reveals that he'd treated Catrina as a child, who had a disease impairing their ability to walk probably. The tonic he made was the only thing to help her, meaning that by Catrina's refusal, Gaius' theory about her true identity was proven. The following morning, Uther decides to take Catrina on a horseback ride before breakfast. Merlin sneaks into her bedroom and is overcome by the foul smell, to be met by Jonas. Merlin notices Jonas' tail and quickly exits the room.
By the river, Catrina appeals to Uther's sympathetic side and reassures him that they will both find love. Gaius approaches Uther about the Lady Catrina and tells him about Catrina's disease but is disregarded by the King. He questions Catrina about her disease and she admits that she did suffer from the sickness but he was mistaken in his thought that it was incurable. Uther rejects Catrina's romantic advance and she leaves in anger, only to have Merlin follow. Merlin steals a mirror from Arthur's bedroom and by performing a spell, controls the mirror to fly out of the mirror until it shows Catrina's true form. Arthur confronts Merlin and assumes that he is spying on Catrina. Merlin rushes out and follows Catrina to her improvised dwelling underground, where he finds her sleeping. Not knowing he has been noticed by Jonas, Merlin returns to Gaius to learn that Catrina and Jonas are after Uther's fortune and power.
Gaius breaks the news to Uther, only to receive a cold response from the King who refuses to believe him. In her chambers, Jonas tells Catrina that Merlin followed her the night before and offers to dispose of him. Catrina refuses his offer but states that it is time Uther finally bowed to her orders. Catrina presents Uther with an enchanted pendant which makes him automatically besotted with her as Uther initially opposes the idea of their relationship.
The next morning, Uther tells Gaius of his love for Catrina as he has asked her to stay in Camelot indefinitely. Gaius and Merlin devise a plan to reveal Catrina's true form, by Merlin using his magical powers in front of the King. Initially opposed to the idea, Merlin admits that he has no knowledge of troll magic and he begins to study. Gaius and Merlin are summoned to the council chambers where Uther and Catrina make the announcement of their engagement and immediate marriage; to the surprise of the crowd and Arthur's shock.
Merlin subtly recites his spell only to be put off by Catrina. She orders Jonas to trap Merlin in order to stop him from ruining their plans. Merlin goes to see Arthur as he is their only hope and tells him about Catrina. Arthur assumes Merlin is joking and simply tells Merlin to get used to the idea of Catrina being Queen. Merlin comes across Jonas in an emotional state and he tells Merlin he is a slave to Catrina, pretending to be abused by his Mistress. Jonas tells Merlin the only way to stop Catrina is to steal the potions she needs to stay human from her dwelling underground.
Merlin does as Jonas suggests and is trapped underground by Catrina. Gaius reaches Merlin's room to find him absent are Merlin realises he the extent to which he is trapped as even his magic cannot break the barrier. As the wedding approaches, the castle prepares for the ceremony. Arthur and Uther first enter the ceremonial chamber as Merlin continues to try to escape. Catrina makes her entrance to a crowd of many as Gaius watches on. In his final attempt of escaping, Merlin finally breaks through his prison and rushes to stop the wedding. As he approaches the chamber, Merlin is confronted by Jonas who refuses to let him pass. Physically apprehending him, Jonas assaults Merlin and is counter-attacked by Merlin's magic. As Uther and Catrina are proclaimed husband and wife, Merlin enters, unable to stop anything.