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.

Cinque and Gaul's duel is interrupted by Leonmichelli who reprimands both for ruining Biscotti's victory party. Almost running out of time for Millhiore's concert, Cinque comes with a plan to return the princess to the castle for her performance.

Cinque learns from Millhiore how she used her stargazing powers to learn about him and why she chose him as her hero. Meanwhile, Leonmichelli confirms by using her own stargazing powers that an ominous fate awaits them.

Desperate to change Millhiore and Cinque's dreadful fate, Leonmichelli challenges Biscotti for another battle with each country waging their most sacred treasures on it. Millhiore reluctantly accepts the challenge, unaware of her true intentions.

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.

Cinque and Millhiore manage to subdue the monster and restore the fox cub to its former self. Yukikaze offers herself to take care of it and a huge concert featuring the princess is held to celebrate the end of the hostilities between Biscotti and Gallete.

Just after Millhiore confesses her love for him, Cinque is sent back home and has no recollecions of his days at Flonyard. However, Ricotta learns that a way to make his memories return and be summoned back is possible, thanks to the possessions he left behind as a keepsake to his friends and the promise he made to them that he would indeed return. Upon regaining his memories, Cinque renews his vow to visit Flonyard again as soon as possible.

The monster grabs Millhiore and flees the fortress on a rampage. Cinque struggles to reach the princess before she is absorbed by the creature, while she learns that the monster is actually a fox deity cub whose body was pierced by a cursed sword.

At last Ricotta and her assistants come with a way for Cinque to return home. However, she finds that by doing so, not only he will forget all memories and experiences of his time at Flonyard, but he will never be able to come back. Upon learning of it, Cinque asks Ricotta to not tell the truth to the others, as he does not want them to send him off with a sad face.