We open on the evil Knight Valiant buying a magical shield embossed with snakes. The sorcerer from whom he buys it enchants the snakes to come alive, they will kill his opponents in battle, but Valiant orders the snakes to kill the sorcerer. They have a new master now.
A week later, Valiant rides into Camelot and enters the annual sword fighting tournament, where he'll face the reigning champion, Prince Arthur. The snakes on his shield are hidden for now but they await only his command to strike.
Meanwhile, a disgruntled Merlin is getting to grips with his many arduous duties as Arthur's new servant. Likewise, Arthur is not exactly impressed with Merlin's lack of talent for his new job.
The tournament is nearing its conclusion. Arthur will fight Valiant in the final. But Merlin realises that Valiant is cheating and soon discovers the magic of the shield. If Valiant unleashes the magical snakes upon him, Arthur will surely die.
Merlin presents his evidence to Arthur, accusing Valiant of using magic to cheat in the tournament. Arthur is initially dubious, but he takes a leap of faith and trusts Merlin. However, we see Valiant is spying on Arthur and Merlin.
The entire royal court watches Arthur make his case to the King, but Arthur's case collapses and he is forced to withdraw the allegations; his father looks on, disappointed. Arthur blames Merlin for the humiliation and sacks him.
Merlin must master the magic that brings the snakes to life so he can expose Valiant and save Arthur, but he struggles with it. It's the day of the tournament final and Merlin's running out of time.
Arthur fights Valiant in the tournament final, dodging snake bites from the magic shield. Merlin finally masters the magic, races to the amphitheatre and uses his magic to expose Valiant by revealing the snakes to the world. It's almost too late - Arthur has lost his sword and the snakes are about to kill - but Morgana throws Arthur a new sword. With a final swipe, he kills the snakes and their evil master, Knight Valiant.
Arthur is crowned champion and his reputation is restored. Arthur re-hires Merlin as his servant and lists all the duties that need doing. Merlin's back to exactly where he started...
Morgana and Mordred prepare to destroy Camelot (and Morgana plots to destroy Emrys). Is Merlin Camelot€™s last hope?
Merlin star Colin Morgan talks about the important events from episode 511 'The Drawing of the Dark.'
Mordred battles between his duty to his friends and that to his love, but is spared the choice.