Following on from the series three opener, savage sorceress Morgause (guest star Emilia Fox) and her fast-learning apprentice Morgana are turning Camelot into less of an enchanted land and more of a hell hole…

Morgause has manipulated the fighting spirit of King Cenred (Tom Ellis – Miranda), who has gone to war against a weakened Camelot… Merlin’s still recovering from the serket’s attack and King Uther is still under the spell of the mandrake root.

In other words, both are about as much use as a flyswatter would be against The Great Dragon. That leaves Arthur, the valiant prince, and his faithful men. But they haven’t got a chance against the enemy within: Morgana.

Morgause has armed Morgana with a rowan staff – a magic wand that does a lot more than pop rabbits out of hats. Morgana raises the dead from the vaults of Camelot and Arthur finds he is fighting not only Cenred’s troops but also a skeleton army.

Camelot is in real danger of defeat; Merlin is their only hope. Of course, he’s supposed to be just Arthur’s manservant; no one knows he’s the world’s greatest wizard in training – except Morgana, Gaius and The Great Dragon. Two of them he can count on as allies – but Morgana has gone to the dark side and doesn’t want Merlin’s magic to save Camelot.