With Merlin gravely ill from the Serket's poison, Morgana and Morgause continue with their evil plan. They have mentally incapacitated Uther and convinced his enemy, Cenred, to invade Camelot. The fate of the kingdom now rests on Arthur's shoulders.

The Prince and his Knights resist the invaders, but Cenred's soldiers are not the only threat they face. Can Merlin stop Morgana in time to save Arthur from the enemy within?