A blast of energy sends the Guardians to the ruined Moon Kingdom, where they meet a hologram of Queen Serenity and learn about their past. Luna and Artemis find the entrance to the Forbidden Mountains. A new enemy, Rothbart, attacks the heroes, but is defeated by Sailor Moon. (Nagisa, Honaka, and Hikari are guest stars in the movie.)
King William (Dakin Matthews), widowed father of newborn Princess Odette, and Queen Uberta (Sandy Duncan), widowed mother of young Prince Derek, decide to betroth their children in the hopes of uniting their kingdoms. For safety William odered Usagi to divide her group, So Usagi, Rini,Nagisa, Honaka and Hikari stays with Derek, while Ami, Raye, Lita, and Mina looks after Odette. Rothbart (Jack Palance) is an evil enchanter who wishes to take William's kingdom for himself, but before he can make his move, he is attacked by William's men and the Sailor Scouts. Although banished from the kingdom, Rothbart vows to return.
William,Uberta and the team have Odette and Derek meet every summer in the hopes that they'll fall in love. As children this fails miserably, but when the years pass and the two reach adulthood, they do fall in love. Derek (Howard McGillin) declares that the wedding preparations begins, but when he expresses his wish to marry Odette (Michelle Nicastro) solely for her beauty, she rejects him for good. Odette, William and the few of the team members leave Uberta's castle, and they are intercepted by Rothbart, who transforms into a "Great Animal" (a mythological creature with the head of a wolf, the body of a bat, the feet of an eagle, and the tail of a lizard), kidnaps them and fatally injures William. Derek (along with Usagi and the others) arrives on the scene, where William tells them with his dying breath that they were attacked by a "Great Animal," and that Odette and the others are "gone." Believing that Odette is dead, Uberta encourages Derek to find another princess, but he is determined to find Odette, while Usagi and her friends try to form a plan to find the others. Derek and his best friend Bromley (Joel McKinnon Miller) practice hunting every day in preparation for facing the Great Animal.
Elsewhere, Rothbart is keeping Odette and the others at his castle lair at Swan Lake. He has cast a spell that turns Odette into a swan during the day, and she is able to temporarily turn human at night if she is on the lake under moonlight. Every night Rothbart asks Odette to marry him so he can rule William's kingdom legally, but she always refuses. During her captivity, she and the others befriends a turtle named Speed (Steven Wright), a frog named Jean-Bob (John Cleese), and Puffin (Steve Vinovich), a puffin bird.
Puffin and Odette, in her swan form, fly together to find Derek. By chance they stumble upon Derek (along with Usagi and the others) in the woods, for they are searching for the Great Animal with Bromley. Derek mistakes Odette for the Great Animal and tries to kill her. The ensuing chase leads Derek and the others to Swan Lake, where they witnesses Odette's change from swan to human when the moon rises. Usagi and the others are happy to find Ami, Raye, Lita, and Mina safe. The two share a loving reunion, and Odette explains that the spell can only be broken by a "vow of everlasting love" that's "proven to the world." Derek invites Odette and the sailor scouts to his mother's ball the following night, in the hopes of declaring to the world of his love for her. Derek leaves just as Rothbart arrives. The enchanter has heard the whole conversation and imprisons Odette as a swan in the castle dungeon, along with Bromley whom he had found in the woods.
Rothbart, worried about Derek's vow, sends his hag sidekick to the ball disguised as Odette. Odette's friends and the sailor scouts free her from the dungeon and she flies to Uberta's castle, but she is unable to warn Derek (along with Usagi, Rini, Nagisa, Honaka, and Hikari) in time. Derek makes the vow to the wrong woman, which causes the spell to start killing Odette. When Usagi wondered where the sailor scouts and felt something wasn't right. The ball is stopped, when Rothbart arrives, Derek realizes his error and races after Odette back to Swan Lake, along with the rest of the team. Odette transforms back into a human just before she dies in Derek's arms. A furious Derek confronts Rothbart, saying he should not let Odette die and is the only one with the power. Rothbart transforms into the Great Animal. A battle ensues with the heroes against Rothbart overpowering Derek and nearly killing him. Odette's animal friends return Derek's longbow to him, and Bromley, who has escaped the dungeon, provides Derek with a single arrow. Derek catches and fires the arrow into Rothbart's heart, killing him. Derek confesses to Odette that he loves her for her kindness and courage, he always had. Odette returns to life, the spell on her broken. Usagi and the team watched as the two are married and live happily ever after.