While out on a routine hunting trip, Arthur and his men encounter a huge and terrifying monster. It has the body of a lion and the head of a serpent, with a huge pair of dripping fangs. Arthur and his men run for their lives. Merlin realises one of the party is missing. A beat, and we hear a man's dying scream echo through the darkness...
Gaius explains that what they encountered was a Questing Beast, said to have been conjured from the nightmares of a long-dead King. It is a creature of great power from the Old Religion, whose bite is always fatal. Uther refuses to acknowledge such folly and sends the Knights out to kill it.
Arthur and his men creep through the forest and nervously follow its trail to a cave. As they cautiously enter its lair, the beast jumps at Arthur. Merlin rushes to help, but doesn't quite get there in time - the beast hurls Arthur to one side. Merlin uses magic to send a sword flying at the creature, and it vaporises before his eyes. The beast is dead, but Arthur lies unconscious on the floor, blood dripping from his shoulder.
Arthur is rushed to Gaius - the physician takes one look at the ailing prince and says there is nothing he can do. Merlin blames himself for failing in his destiny to protect the Prince. He tries to use magic to save Arthur, but it doesn't work. Gaius tells him that the bite of the Questing Beast is a death sentence which no magic can overturn.
The castle goes into mourning, but Merlin refuses to give up; he seeks the help of the Great Dragon. The Dragon tells him he must go to the Isle of the Blessed and ask the old religion to save Arthur - it alone has the power.
Merlin goes back to Gaius with the clues the dragon has given him. Gaius fills in the gaps - the ancient magic which has the power to mirror life can reverse death itself, but there will be a price to pay.
Merlin journeys to the Isle of the Blessed, where he finds Nimueh waiting for him. Nimueh tells Merlin that she can save Arthur's life, but he must know that a price will have to be paid. The balance of the world must be restored. A desperate Merlin says that he will take that chance, Arthur's life is worth anything. If he has to give up his own, then so be it. Nimueh shows him a gold chalice - it is the Cup of Life, blessed by centuries of powerful sorcery so that it contains the very secret of life itself. She tells him that if Arthur drinks from the cup, he will recover.
Merlin feeds Arthur the water from the chalice - he slowly shows signs of recovery. Gaius declares he will live. The castle rejoices that their Prince is recovered - and Merlin awaits his fate - but when his mother, Hunith, arrives and is gravely ill, he is stunned. This was not the bargain he thought he had struck.
A furious Merlin crashes into the dragon's cave. He accuses the creature of setting him up. The dragon does not deny it. He tells Merlin that Arthur must live and his father must die so the ancient forces can exist again in the realm. Merlin is furious - Arthur must live at the expense of his mother - the cruellest trick of all, and all so that the dragon can get what he wants, to be freed from his prison. Well, Arthur will live, but Uther will also live on, to spite the dragon and keep him hidden in darkness. Merlin will make sure of that. He tells the dragon that his visits are at an end and leaves the furious monster breathing fire around his darkened prison.
Merlin is distraught, and knows what he must do. He tells Gaius if the balance of the world needs a life, it must take his. He will return to the island and give his life in Hunith's place. An anguished Gaius tries to dissuade him - Merlin's talents, his destiny, are too precious to be sacrificed - but Merlin refuses to listen.
He goes to Arthur. He tells Arthur that he is both a great warrior and a good friend and because of these two things he will make a great King - he must always remember to listen as well as to fight. Arthur has no idea what he is saying - but we know that Merlin is saying goodbye.
Returning home to collect his things, Merlin finds a letter. It tells him that Gaius has gone to the Isle of the Blessed in his place. Gaius lovingly acknowledges that Merlin's life is worth more than his and it is his that should be given instead. A distraught Merlin thunders out of the gates after Gaius.
Gaius makes his way out to the island on the lake and confronts Nimueh. He offers his life in Merlin's place. He makes a great speech about how he believes in the world that Arthur and Merlin will create and he is willing to give his life for the future they will bring.
Merlin finally reaches the Isle of the Blessed. He finds Nimueh in the process of conducting a ceremony on Gaius to take his life. Filled with rage, Merlin conjures a mighty bolt of magic and launches it at Nimueh. The two great sorcerers duel. Merlin has to draw deep within himself - but the sight of Gaius' slumped body gives him an anger we have never seen in the young warlock before. Finally he strikes Nimueh down with a mighty blow.
Merlin rushes to Gaius' side immediately - is his friend and mentor really gone forever? He cradles his friends head in his hands and just as he begins to give up hope, Gaius begins to come around. He opens his eyes and Merlin is ecstatic.
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