Arthur is a reluctant king, nervous about his upcoming arranged marriage and regretting that he ever pulled Excalibur from the stone. Morgana, his half-sister, covets his crown. She casts a spell which will cause Excalibur to break the next time he touches. Meanwhile, Arthur’s greatest knights—Lancelot the Inexplicable and Gawain the Boastful—prepare the audience for the upcoming celebration of Arthur’s one-year anniversary as king. When Gawain goads Arthur into using his sword as a fly-swatter, it breaks. The three of them decide to visit Merlin, to see if he can fix it.
Along the road to Camelot, Princess Guinevere and her nursemaid, Dame Elaine, pause to catch their breath. Guinevere is nervous about marrying a total stranger and wants to know what Arthur is really like. She convinces Elaine to switch places with her, so she can pretend to be a servant and observe from the sidelines.
When the knights visit Merlin, he’s unable to fix Excalibur with super-glue. Morgana arrives on the scene, supplying Arthur with a phony “king’s handbook,” which states that he must reforge the sword in three days, in the waters of the Lake of Avalon. In order to approach the waters, however, he must perform three kingly miracles. The knights head out and immediately encounter Guinevere and Elaine along the side of the road. Lancelot is instantly taken with Elaine, who he and the others mistake for Guinevere. Meanwhile, Arthur and the real Guinevere snarl and argue with each other, especially when Guinevere insists that they come along on the adventure.
Disguised as a beggar woman, Morgana tricks Arthur into believing that she’s dying. The only cure is the hair of a unicorn. As it happens healing the sick is a kingly miracle, as is rescuing a damsel. They’re dangerous creatures. Fortunately for Arthur, Guinevere again insists on tagging along. While no man may touch a unicorn, the creature is putty in Guinevere’s hands. The two of them begin to like each other, especially when Arthur reveals his own reluctance to marry “Guinevere,” not because she isn’t what he expected, but because he doesn’t really know her.
When Arthur and Guinevere return with the unicorn hair, foiling Morgana’s plan to have Arthur killed, Morgana reveals herself and kidnaps Guinevere, leaving Arthur, Lancelot, Gawain, and Elaine to mount a rescue. She offers Guinevere the chance to join with her and rule the kingdom side-by-side, but Guinevere, who’s come to admire Arthur, refuses. Morgana then takes Guinevere to the Forest of the Siren, to use her as bait to lure Arthur to his death. Arthur and his friends are transported to Guinevere’s location by Merlin and immediately fall into the trap, the Siren drawing the men away. Immune, Guinevere and Elaine break the spell, for Arthur because he hears Guinevere’s voice and loves her, for Lancelot because he hears Elaine’s voice and lovers her, and for Gawain with his reflection, because he loves himself. Meanwhile, Merlin challenges Morgana to a magical duel and manages to best her.
All of the characters meet up at the Lake of Avalon. Arthur allows Merlin to use the unicorn hair to cure his headache. Thus, he’s accomplished two miracles: Rescuing a damsel and healing the sick. Guinevere reveals that he’s also won the heart of a princess, because she’s the real Princess Guinevere. That’s three miracles. Arthur is able to restore Excalibur. Everyone returns to Camelot for a double wedding, Arthur and Guinevere, Lancelot and Elaine.