Cinque Izumi makes a promise with his best friend Rebecca to go flower viewing during the last three days of Spring Break. However, while he is alone trying out some acrobatic moves in school, a dog appears and opens a magical portal while Cinque is in mid-air, resulting in him jumping through the portal with the dog. He arrives in the land of Flonyard, and is greeted by a beautiful girl, albeit with bestial features like dog ears and tail. The girl, who introduces herself as Princess Millhiore, requests for his assistance, which he agrees to and becomes the Princess' Hero, in order to drive away the forces of Galette which seeks to invade Biscotti.

Biscotti's army fights a losing battle with the forces of Galette until Cinque manages to hold off the enemy's offensive fighting alongside Captain Éclair Martinozzi. However, the two must join forces to confront Galette's commander and most powerful warrior, Leonmichelli Galette des Rois.

The battle ends with Biscotti's victory, but Cinque learns that Millhiore was not aware that once he was summoned to Flonyard, he cannot return home. Meanwhile, Leonmichelli's younger brother, Gaul, learns of his sister's defeat and is eager to face Cinque in combat.

Gaul's special forces, the Génoise trio, kidnaps Millhiore and he challenges Cinque to a duel. Cinque, Éclair and chief researcher Ricotta Elmar come with a plan to storm the fortress where she is being held to rescue her in time for her victory concert. Heavily outnumbered by the enemy, Cinque and Éclair manage to turn the tides when the freelance knights Brioche d'Arquien and Yukikaze Panettone come to assist them.

The day of the battle comes and it does not take long for Millhiore to confirm her suspicions about Leonmichelli's behavior and she joins Cinque and Éclair to storm the fortress where Leonmichelli is stationed to confront her. Meanwhile, Brioche and Yukikaze feel that a strange presence is approaching.

Bearing both sacred swords, Millhiore reaches the top of the fortress to confront Leonmichelli, who trembles when she realizes that her actions are leading the princess to the same destiny she fought so much to avert, when a powerful and ominous creature appears before them.