When Alice, an old flame of Gaius's arrives in Camelot, Merlin is pleased to see a different side to his mentor but after discovering that she is under the influence of a Manticore, a creature from the Old Religion, Merlin has no choice but to tell Gaius of Alice's evil intentions. Blinded by love, the physician refuses to believe Merlin's story and the two friends fall out. With the King in danger and all evidence pointing to Alice and the evil magical creature that has possessed her, Merlin must somehow make Gaius come to his senses before it is too late.