Travelling to a distant village in the hope of finding a cure, the young warlock is soon lured into a deadly game of cat and mouse. But is he the hunter or the prey?
And who exactly is the mysterious Lamia (Charlene McKenna)? Could she hold the key to unlocking this latest mystery?
> Merlin: 'The Secret Sharer' recap
Take a look at some pictures of guest star McKenna in character below - Merlin continues this Saturday at 8.15pm on BBC One.