Uther is captivated by his latest guest, the beautiful Lady Catrina. Arthur is appalled by his father's flirting, but Merlin is wary for a different reason - he suspects she is really a troll!

Catrina and her strange servant are welcomed into the heart of Camelot, and it is up to Merlin to prove she is a monster and save Uther from ruin. Unfortunately, Catrina has her sights set on a far greater prize than the king's heart - can Merlin overcome her powerful troll magic before it is too late?

Uther is determined to wipe out magic in Camelot once and for all, and this time no one is beyond suspicion. He summons the most dreaded witchfinder in the land, Aredian, a terrifying man who will stop at nothing to uncover sorcery. Gaius impresses upon Merlin how much danger he is in but Morgana needs no such warning. She is certain Aredian will see straight through her and she will be burnt at the stake. Can Merlin protect Morgana and Gaius without ending up in the flames himself?