Briefly, the legend avers that whomsoever should sleep in the keep room would sleep for fifty years, or awaken in the morning to find his road clear to a kingship. Lord Plantoro, coming to wed Guinevere, daughter of Lord Sterling, hears this ancient legend chanted by minstrels at the betrothal banquet and vows to sleep in the keep. He does so, but Guinevere in the night, with the aid of friends, changes things in the room to give an appearance of great age. Disguises in the morning help carry out the joke. Planoro's mind is unhinged, however, and the Mitred Abbot, Benedict, compels Guinevere to consecrate her life to him at the altar. Years later, however, in a beautiful scene, his mind is restored in a novel manner. This is only a bare suggestion of the story, however. Its beauty and strength can be appreciated only in the viewing of it.
—Moving Picture World synopsis