In a dark chamber, Morgause incants a powerful spell over seven dusty old statues of knights. For the first time in over three hundred years, the suits of armor begin to stir.
A farmer arrives in Camelot to report smoke over the Fortress of Idirsholas. Gaius tells Merlin that, over three hundred years ago, the seven noble Knights of Medhir were seduced by a sorcerer. As Uther bids Arthur to ride out and confirm that all is well in Irdisholas, Gaius warns Merlin not to be complacent - if someone has revived the Knights, then Camelot is under threat.
As Arthur, Merlin and a troupe of knights ride out, Morgana is visited at her window by a hawk carrying a message from Morgause, asking Morgana to meet her. That night, Morgana sneaks out of the castle and when she greets Morgause it is clear that the strange bond between the two women is as strong as ever. Morgause asks Morgana whose side she is truly on. Morgana insists she will help bring down Uther in any way she can. Pleased, Morgause gently sends Morgana to sleep and utters a powerful spell over her body.
The next day, Merlin and Arthur arrive at the Fortress of Idirsholas. At first, it appears empty but as the group make their way through the sinister hallways of the fortress, Merlin and Arthur are set upon by the Knights of Medhir. Swords are drawn and an epic fight ensues. Merlin uses magic to protect Arthur, but our heroes barely escape with their lives.
When Merlin and Arthur return to Camelot, they find the town deadly still. Every guard, knight, townsman and noblewoman is slumped over in a deep sleep. The boys enter the castle and discover it's the same there. It is clearly the work of magic, but who could have done such a thing, and why? Merlin and Arthur discover there is one person still awake - Morgana. They press her for answers, but she's terrified and confused.
As the Knights approach the castle, Merlin, Arthur and Morgana finally find Uther, slumped in deep sleep in a private chamber. Merlin makes an excuse to leave and desperately searches through his spell book for a cure. He tries one spell after another on a snoozing Gaius, but none work.
Desperate, Merlin goes to the Great Dragon, swearing on his mother's life that he will honor his promise to free the captive beast. The Dragon tells Merlin that powerful sleeping spells need a constant living presence to give them strength. The source of Camelot's pestilence is the witch, Morgana, and the only way to end the magic is to kill her. Merlin is horrified, but the Dragon is firm. In shock, and still uncertain, Merlin grabs a bottle of poison from Gaius' supplies.
As the Knights of Medhir enter the castle, Arthur decides the only hope it to get Uther out of the palace. They struggle to drag him through the corridors but the sleeping spell begins to affect the boys, too. Arthur bravely takes the Knights on singlehanded, ordering the others to protect his father. Morgana learns that Morgause is in the castle and her suspicions are confirmed - this is her fault. Arthur steels himself; he will carry on till the end if it gives Merlin and Morgana any chance of getting his father to safety. Ignoring Merlin's pleas, Arthur heads out to make a last stand. And that's when Merlin makes his terrible decision...
He adds the poison to a water bottle and offers it to Morgana. Morgana drinks, and almost instantly begins to fade. Outside the room, Morgause can sense the life draining from her sister, and frantically battles into the room. As Arthur turns his fight back to the Knights, demands that Merlin reveal how he poisoned Morgana, but Merlin won't do so unless Morgause undoes the magic driving the Knights. Stalemate. Who will back down?
Arthur is disarmed and pinned to the wall by one of the Knights of Medhir - it looks like the end. Suddenly, another sword parries the Knight's blow! Arthur turns to face...Sir Leon. His men have started to wake!
Merlin hands Morgause the empty poison bottle and questions why she cares so much about Morgana. Emotional Morgause admits they shared a mother; Morgana is her half sister. The Knights of Medhir crumple to the floor and Arthur and his men charge into the council chambers just as Morgause weaves a powerful spell. When the air clears, Morgause has gone...and so has Morgana.
The Great Dragon is pleased to see the young warlock. A nervous Merlin asks the Dragon to promise he will not harm Camelot once he is free of his chains. The Dragon refuses to make any more bargains. Though uncertain, Merlin knows he must keep his promise. He raises a sword of the Knights of Medhir, incants a spell, and brings the blade down on the Dragon's chains. Merlin watches as the Dragon flies up, up and away, free at last.
Merlin has pushed the limits of his strength to save Arthur, but even he may not be powerful enough to rescue Camelot€™s king.
In the battle foretold by the prophecies, all give their best, and Merlin wins the day.
Morgana may have one last trick up her sleeve to win the war.