Arthur relays a message to his father that an entire Camelot patrol was massacred in the Forest of Gedney. Uther is furious to hear that his old foe Cenred ordered this because the patrol had trespassed on his land. Meanwhile, an injured Sir Leon is taken back a cave by Druids. The Druids gather round and chant, and Isledir, an elder, brings a golden cup from which Sir Leon takes a sip. His wounds are healed.
Sir Leon returns to Camelot, and Uther is curious at how Sir Leon managed to recover so miraculously. Nervously, Sir Leon tells of the life-giving cup. Uther hastily breaks up the council meeting, and Morgana leaves with a curious look over her shoulder. Later, Gaius tells Merlin that the Cup was used to create an army of immortal soldiers. If it were to fall into the wrong hands, Camelot would be all but lost.
Uther orders Arthur to retrieve the cup, but insists Arthur only take Merlin with him. He swears Arthur to secrecy - not even Merlin can know where they are headed. At the same time, Morgana tells Margause of the Cup of Life. Although unsure of the exact location, Morgana knows Arthur is heading towards Cenred's kingdom.
As Merlin and Arthur engage in their quest, Merlin worries about the dangers ahead. Arthur tries to reassure him, but speaks too soon - a poison dart pierces Arthur's neck. Merlin is hit quickly after, and both fall unconscious.
The pair wakes in a dungeon, alongside questionable characters. To their surprise, they also find Gwaine, who tells them an evil slave trader named Jarl is holding them captive.
Jarl enters the pit looking for someone to fight his champion. No volunteers lead Jarl to choose the opponent: Merlin. Arthur offers to take Merlin's place, and Jarl agrees under one condition - it must be a good fight, or he'll kill them all. Arthur agrees and learns that Jarl's champion is Gwaine.
Arthur and Gwaine pretend to fight, but to no avail. Merlin realizes he must do something, and his eyes flash. A torch's flames shoot into the sky, and the trio make their escape in the commotion.
Jarl's men search Arthur's confiscated possessions and find his crest. Jarl goes to see Cenred to sell the Prince's whereabouts. As he waits for his reward, Morgause's eyes flash; Jarl flies against the wall, dead.
On the road, Merlin lets slip to Gwain about the Cup of Life. Although furious, Arthur concedes Gwaine's help will be invaluable. The trio find the Druid cave. Something stirs in the shadows, and Arthur finds a young boy in the darkness. Arthur demands information from the boy, but he is not alone. Dozens of Druids emerge; they tell Arthur if he is seeking the Cup, he can take it. As the trio leave with the Cup, Merlin hears the voice of Isledir warning him that the future of the land depends on Merlin keeping the Cup safe.
As they head back to Camelot, the trio is ambushed by Cenred's men. Arthur is hit by a crossbow and the Cup is taken. Rather than pursue the Cup, Merlin decides to stay with his injured friend. Cenred and Morgause finally have their prize. Morgause casts a spell and takes a drop of blood from each member of Cenred's army. Her Immortal Army is born.
Cenred proclaims the Immortal Army as his own, but Morgause is quick to correct him. He tries to protest, but Morgause's magic turns one of the King's own men on him.
Arthur wakes from the fever and realizes the Cup is gone. He is in pain but demands the team press on. The trio passes through burning villages on their way to Camelot. Merlin notices no enemies have been killed; they realize the army cannot die. The Druids' warning has come true.
In Camelot, dead bodies fill Lower Town. Arthur enters Gwen's house and is attacked by a fearful Elyan. Elyan tells them Morgause's Immortal Army has taken the citadel. Anyone found outside will be killed. Arthur ignores the warnings and continues on to find his father, Elyan and company in tow.
Merlin and Arthur seek Gaius for medical help and find him terrified inside a cupboard. After Gaius cures Arthur temporarily, Gwain and Elyan bring news that Uther has been taken to the throne room. Arthur orders the rest of the group back to the woods for safety, but Merlin stays. Merlin's reasoning is simple: Arthur's destiny is too great to abandon him.
The pair sneak onto the throne room balcony and see Uther at the feet of Morgause. As he protests her claim to the throne, Morgana steps out from the shadows. As Morgana is a legitimate heir, Uther and the duo are helpless as she sits on the throne. Geoffrey of Monmouth reluctantly crowns her Queen of Camelot.
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