Four kings arrive for peace talks. Among them are King Olaf and his daughter Vivian, and King Alined and his jester Trickler. On Alined's instruction, Trickler enchants Arthur, making him fall hopelessly in "love" with the beautiful but loathsome Vivian. King Olaf is very protective of his only daughter and will not take kindly to a potential suitor.
The spell-bound prince forgets Guinevere and encourages Merlin to give flowers to 'his love'. Merlin is bemused by this request but agrees and - assuming Arthur is referring to Gwen - leaves the gift on her kitchen table. Merlin soon realizes his mistake when Arthur enthuses about Lady Vivian. He tries to retrieve the flowers, but Gwen gets there first and is clearly touched by the Prince's gesture.
Merlin is increasingly concerned by Arthur's extreme feelings for Vivian, especially when Gaius warns that conflict between Uther and Olaf will certainly bring an end to the peace talks. He finds a lock of Vivian's hair under Arthur's pillow, and Gaius concludes that the Prince has been enchanted. Merlin knows he must find a spell to return Arthur to normal before things get out of hand. Luckily, one thing is on their side - Vivian has no interest in Arthur at all! Frustrated, Alined orders Trickler to enchant Vivian as well.
The Lady Vivian falls prey to Trickler's magic. She arrives, wildly amorous, at the door to Arthur's chambers. As both Arthur and a suspicious Olaf approach the room, Merlin uses a sleeping spell on Vivian and hides her in the wardrobe. Olaf arrives, worried about his missing daughter. He accuses Arthur of concealing her in his room but, thanks to Merlin magically sealing the wardrobe shut, she remains hidden.
Gaius and Merlin are hopeful that Vivian will remain asleep until the peace treaty has been signed, but things continue to spiral out of control. A note is slipped under Arthur's door, inviting the prince to a sunset rendezvous. Gwen has plucked up the courage to return Arthur's gesture, but he assumes the note must be from Vivian.
As the momentous peace treaty is agreed upon, Gwen nervously prepares herself for Arthur's arrival, and Arthur scales the palace walls to reach the bedroom of 'his love.'
With both Arthur and Vivian suspiciously absent from the official peace celebrations, Olaf's blood begins to boil. He sets out for his daughter's room. Merlin arrives just as Arthur reaches sleeping Vivian's bedside and moves in for a kiss. Merlin desperately tries and retries a spell to undo the love potion but it doesn't work. Arthur and Vivian kiss just as King Olaf rounds the corner.
Olaf takes in the scene. Furious, he throws down his glove - a challenge! Arthur would rather die than deny his love for Vivian, so he accepts a fight to the death.
No matter who wins the challenge, someone will die and a renewal of war is inevitable. Uther is appalled, but Arthur cannot be convinced to withdraw. And Gaius won't let Merlin reveal that the whole thing was orchestrated by Alined, because if Uther knew magic was used in his kingdom by a rival then war would surely break out again.
As Arthur and Olaf prepare for battle, Gwen is heartbroken. Arthur and Olaf fight first with the quarter staff. Distracted by the sight of Vivian in the stands, Arthur does not perform well. He is struck hard in the middle, and breaks a rib. The sand drains from the timer - round one to King Olaf.
Gaius sends Merlin down to consult the Great Dragon. Unless the love spell is somehow broken, Arthur will surely lose the challenge and be killed. The Dragon tells Merlin that only one thing can possibly break the enchantment - a kiss from the person Arthur truly loves.
Round two - a mace fight. Arthur is once again soundly thrashed, this time injuring his shoulder. Olaf is about to deliver the killing blow when the sand runs out. They will fight another round.
Merlin tries to convince Gwen that Arthur needs her help but she declines. Arthur has made it perfectly clear how he feels about her. Merlin explains Arthur is enchanted, and he begs her to come with him and help break the spell. Merlin and Gwen arrive to find Gaius tending to Arthur's wounds. He's terribly hurt, but seems to feel nothing but dreamy desire for Vivian. Merlin quickly leaves the prince and Gwen alone. Arthur is puzzled by her presence until she kisses him. Arthur is surprised at first, but then sinks into the kiss. The spell is broken. Gwen has to inform Arthur that he's in the middle of a fight to the death - and he's losing.
The next round is a swordfight. At first, it seems Arthur is far too injured to defend himself, let alone prevail. But just as all is lost, he looks up to see Gwen in the stands. The pair lock eyes and Arthur finds a sudden burst of energy. He throws Olaf to the ground, victorious, and stands poised to run him through...but then steps back and extends a hand, calling for peace.
Vivian is more delighted with Arthur than ever, but he only has eyes for Gwen. Olaf and Uther reconcile, and the peace delegation packs up and moves out of Camelot. Alined's greedy, warmongering plans have failed, and as he departs he takes it out on poor Trickler.
Arthur visits Gwen and apologizes for his behavior towards her when enchanted. He promises he loves only her, but she knows that such talk helps no one because she can never be his queen. After a brief, bittersweet kiss, Gwen moves off, once more simply a servant to her prince.
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