Merlin is troubled by his vision of Mordred slaying Arthur, and his vision wars with his affection for the fellow sorcerer as they depart together with Arthur to avenge the death of a knight. In a confrontation with the murderer, a sorcerer named Osgar, two knights are wounded and he is mortally wounded. He crawls to Arthur, and with his last breath tells the king that The Disir, the priestesses of the Triple Goddess of the Old Religion, have spoken that deity's judgment on Arthur's life. He hands Arthur a large coin with strange engravings, and expires.
Gaius warns, as sternly as he may without revealing his nature as a sorcerer, that The Disir and the coin - actually a talisman called a rhunemark - should not be brushed aside. Arthur determines to visit The Disir in their sacred place to demand a fair trial, but they know all before he arrives, and are offended that he and his knights enter the holy space with weapons and arrogance. With a look, one of the priestesses hurls a spear at Arthur's heart, but Mordred leaps between it and its intended mark. The knights see they are bested by the power of these women, and retreat to safety.
Arthur begs Merlin to attend to Mordred, but the wizard, who was warned recently by the dragon about the peril Mordred represents to Arthur, feigns inability and insists that they bring the boy back to Camelot to Gaius's care. Once there, Mordred's chances are slim due to the effect of magic coursing through his veins. Arthur realizes that the only way to save his dear friend - a knight who saved his life twice - is to appeal to the magical sisters directly.
When he and Merlin reach the sacred cave, Arthur enters humbly, unarmed, and pleads for the life of his friend. But The Disir demand a high price in return for Arthur's wishes: He must bow to the goddess. They give him until dawn to decide if he will cease his war on the old religion. By firelight, Arthur solicits Merlin's council, and though the sorcerer's heart aches with the desire to live in a world where magic is revered, he cannot let Mordred live. He condemns magic to Arthur, and takes cold comfort in the notion that he is guiding his king toward a greater destiny.
The following day, Arthur and Merlin return to Camelot and receive a warm greeting on the castle steps by a fully recovered Mordred. Merlin realizes he took the wrong path: In denying The Disir their request, Arthur gave them permission to release Mordred from his death sentence. By denying magic, Merlin has sealed Arthur's doom.