With King Uther's sudden death, chaos threatens to engulf Britain. The sorcerer Merlin has visions of a dark future and installs young and impetuous Arthur, Uther's unknown son and heir raised as a commoner, as the new king. The two install themselves in Castle Camelot with their allies, which include Arthur's biological mother, his foster brother, and a quartet of loyal warriors. From Camelot, Arthur tries to build a new and better Britain, where people can live in peace.

Meanwhile, Arthur's cold half-sister Morgan plots to take the crown from him. Banished by Uther, who was responsible for her mother's murder to put Arthur's mother on the throne, Morgan is responsible for Uther's death and wants to rule as his successor. Aided by her loyal maid Vivian and the devious nun Sybil, Morgan takes up residence in Castle Pendragon, from where she schemes against Arthur.