Arthur and Merlin are exhausted as they make their way across the icy plains toward the fortress of Ismere, but with each tired step, the warlock's fear intensifies. What powerful secrets are Morgana and the druid Ruadan searching for among its twisted catacombs? And what dangerous game is Mordred playing? It seems not even Merlin can stop his king from walking into certain danger as the great battle for Albion begins. (TV Guide UK)