Merlin is a young wizard who arrives in the kingdom of Camelot after his mother arranges for him to stay with the court physician, Gaius. He discovers that the king, Uther Pendragon, has outlawed magic and imprisoned the last great dragon deep under the kingdom. The dragon tells Merlin that he plays an important role: to protect Uther's son, Arthur, who will bring forth a great kingdom. When Merlin meets Arthur, he believes that he is an arrogant bully and Arthur, likewise, has a less than stellar opinion of Merlin. Merlin must keep his magic secret or face being executed by Uther.
Morgana is Uther's ward with the power of prophetic foresight, which frightens her and which she keeps secret. Guinevere, whom she calls "Gwen", is her maid and close friend. The first series focused on the development of Merlin and his friendship with Arthur,while the second series focused more on the development of the other cast members. Some of the later series' central themes are Morgana's struggle to control her powers and her eventual corruption to evil because of Uther's actions, as well as the maturation of Arthur to become the great king of legend and his blossoming romance with Gwen. A year later, Morgana returns darker and more powerful than ever before. At the start of the fourth season, she learns about Merlin's identity as Emrys (though she doesn't know that Merlin and Emrys are one and the same) and Merlin finds himself powerless against the spirits of the dead that Morgana summons. Arthur eventually becomes king of Camelot after Uther is killed when protecting him from an assassin.
Other characters from Arthurian legend have also appeared. Lancelot longed to become a knight but was unable to do so because of his birth as a commoner. Mordred has appeared as a Druid boy who formed a bond with Morgana (in the legend he is her son and, according to the legend, either Arthur's illegitimate son or nephew), and it has been predicted by the Great Dragon that Mordred will cause Arthur's death (in the legend this occurred at the Battle of Camlann). Mordred and occasionally the Great Dragon refers to Merlin by his Druidic name, Emrys (the Welsh form of Ambrosius);Ambrosius Aurelianus was a historical figure that Geoffrey of Monmouth partially merged with Merlin. Excalibur is an ordinary sword that becomes enchanted by the Great Dragon and is later thrown into a lake (in the legends, this was the sword Nimueh gave to Arthur).
After an attack on Camelot by the corrupted Morgana and her sister Morgause, the sword is struck into a rock by Merlin to keep it out of mortal hands.