Regular cast members for the first series include Colin Morgan, Bradley James, Katie McGrath, Angel Coulby, Anthony Head, Richard Wilson, and John Hurt as the voice of the Great Dragon.

The teenaged warlock Merlin is sent to the bustling kingdom of Camelot by his mother, to work as an apprentice under the tutorage of Gaius, the court physician.

That night, Merlin hears a mysterious voice calling him and he discovers Kilgharrah, a great and powerful Dragon, bound and shackled down in the dungeons by Uther.

While Camelot prepares to celebrate peace with Mercia's visiting ruler King Bayard, a priestess named Nimueh forges a poisoned chalice and replaces it with one due to be given to Arthur.

Bayard is framed and imprisoned, whilst Gaius deduces the only antidote is from an incredibly rare flower, which only grows in certain caves.

Learning Lancelot's dream is to become a knight and grateful for his rescue, Merlin brings him to court and magically forges proof of nobility so that he can apply.

Now that the sword has been wielded by someone other than Arthur, it is capable of being used for evil; to prevent that, the Great Dragon instructs Merlin to hide Excalibur far from the reach of mortal men.