When the fires of Idirsholas burn for the first time in 300 years, Gaius fears a sorcerer has reformed the deadly Knights of Medhir. Arthur and Merlin ride out to face the Knights ... and barely escape with their lives. Upon returning home, they find everyone in Camelot is under a strange enchantment - all except for Morgana. Could she be behind the magic which threatens the Kingdom? As the Knights attack a defenceless Camelot, Merlin's loyalties are tested to the limit.