When young King Arthur's sword breaks in battle, Merlin leads him to an enchanted lake. There, from waters as still as a mirror, the mysterious Lady of the Lake offers him Excalibur, the most famous sword in all of legend.

On the plain of Camlann, two armies meet for the last battle. Arthur faces Mordred, Excalibur returns to the lake, and a boat crosses the mist toward Avalon. The end of the Arthurian legends — and the promise of the once and future king.