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?