A deadly plague spreads through Camelot. When it seems that sorcery can be the only possible explanation, Merlin and Gaius race to find a cure, while Gwen is arrested and sentenced to death for practising witchcraft.

With her sights set firmly on Merlin, Nimueh tricks him into drinking from a poisoned chalice, and he falls dangerously ill. Arthur ventures forth deep into the forest of Balor to find the antidote, but little does he know that he's walking into a trap