When Morgana's nightmares spill into waking hours, she becomes terrified that she has magical powers. Gaius is determined to keep her in the dark for her own good but Merlin won't stand by and watch his friend suffer. He knows exactly what she's going through and he knows she needs the truth. Is Merlin about to reveal his secret at last?

Wilfully disobeying Gaius, Merlin risks everything to help Morgana, not knowing what the far-reaching consequences might be. Lives are put in danger, loyalties are tested, and Morgana is reunited with the strange young druid boy Mordred, who the dragon predicted would one day kill Arthur.

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?