Petite Princess Yucie follows the adventures of country-girl Yucie as she is admitted by chance to the prestigious Princess Academy, where the daughters of royalty and nobles attend to learn magic, dance, etiquette, defense, art and music. There, she experiences many things in her quest to collect the "fragments" of the Eternal Tiara in hopes that she may become the legendary Platinum Princess, who is chosen every 1,000 years. Yucie, along with the four other Princess candidates who are initially her rivals but are won over by her offer of friendship, must grow in heart--if not in height--to become worthy of the Tiara. Yucie is a spunky heroine who is a genius of smiles, and who, despite her common lifestyle, is actually the daughter of a noble and former hero who has retreated from courtly life and lives in the countryside.
The fragments take the form of Crystal Flowers and are scattered throughout the five worlds; Human, Demon, Heaven, Spirit, and Fairy. Strangely enough, the five candidates Yucie, Glenda, Elmina, Cocoloo and Beth are representatives of these five existing worlds in this story's universe. As the legend goes, once the Eternal Tiara is complete, it will select the Princess from the worthy candidates and can grant her one wish. Unknown to the candidates and the viewers, until almost the very end, is the sad history of the Eternal Tiara and the inevitable fate of the Platinum Princess and all the other candidates.
Blindly pursuing what is her heart's life-long desire, to finally grow up and be treated and respected like an adult, Yucie must prove her worthiness to the Tiara and sets out to do many odd jobs as part of the Academy's special Curriculum (NOTE: community service) for the candidates, such as tending flowers, babysitting a giant fluff ball, helping to run an old church, helping to run a bakery, and overseeing a kindergarten picnic. Each task seems easy at first but problems abound in every episode. The girls must overcome all obstacles and complete their tasks, no matter how daunting, in their petite bodies. The viewers later discover that all the candidates are under the same 10 year old "curse" and their rivalry quickly turns into admiration for one another's determination in reaching their common goal. Yucie and her friends each have a reason for desperately wanting to become the Platinum Princess.
In a side-story arc, Yucie is also hoping to find clues about the lost prince of the Human realm, who once saved her life while she was lost in the forest, looking for Sunset Blossoms, and whom she wishes to meet in person some day to thank him. Her memory of him is vague, and he was an adolescent at the time, so she has no idea what he may look like now or what kind of person he's become. When she encounters a young man named Arc during one of her odd jobs who looks so familiar to her, she has conflicted feelings. Arc is rude, patronizing, unfeeling and insensitive but she feels drawn to him somehow. The prince was her childhood romantic ideal, but her seventeen-year-old heart falls for Arc. To the relief of viewers who hate love triangles, Arc is actually Prince Arrow in disguise.
As the girls continue to grow emotionally, their Platinum Princess candidate pendants grow in beauty and brightness as well, reflecting how much wisdom they have obtained. Towards the final story arc, Yucie and the others finally realize why the Fairy World princess was so troubled and uncooperative when Beth reveals that her father, King of the Fairy Realm, is dying. The girls are resolved in helping their friend and saving the Fairy Realm by defeating the ancient evil Diablo who threatens the existence of all the worlds. With the combined help of their stewards and friends, the girls succeed with the power they obtained as candidates, but the prince is badly injured.
The final story arc begins with the final test in which a magical judge will oversee the Tiara's choice. Yucie’s only wish now is to save the prince with the power of the Tiara.
Upon entering the place of judgment, Yucie and her friends meet a mysterious figure, wrapped in a tattered brown cloak. The judge's identity is hinted at when she uses her magic staff to change all of the candidates' clothing similar to her own. Out of friendship for Yucie, and to support her wish to save Prince Arrow, Glenda, Cocoloo, Elmina, and Beth all renounce their candidacy, making Yucie the only choice. The Tiara accepts Yucie as its new master. However, in one final twist, the girls realize that those not chosen by the Tiara will simply vanish. Yucie refuses to accept the Tiara when she learns this, and tells the others she is determined to save the Prince some other way, as she cannot sacrifice her friends in the process.
After careful thought, Glenda, Cocoloo, Elmina, and Beth ask the judge to erase Yucie's memories of them, so that she can return to the Human Realm in time to save the prince. In a tearful final goodbye, Glenda, perhaps Yucie's strongest rival, tells her to never forget to be her cheerful self. After Yucie leaves, the remaining four sketch a portrait of themselves and Yucie on a stone slab, in remembrance of their friendship, and disappear.
Yucie returns to save Prince Arrow and collapses from exhaustion. The Eternal Tiara disappears after its owner makes her wish, its immense power quelled by Yucie's strength of heart, and history repeats itself. Yucie awakens to the keen sense that she has lost something of great importance to her, but cannot remember what. Her father, Gunbard, feels responsible for her misery, since he was the one who put the Tiara together and restarted the Legend in their lifetime. With the help of the fathers of the missing girls, he opens the gateway to the place of judgment so Yucie and Prince Arrow can retrieve what was lost.
Yucie faces the cloaked figure once more and demands that what was lost be returned. The viewers discover that the ruined empty world that was the place of judgment is actually the lost Magic World, a sixth realm in this story's universe. The judge was actually a Platinum Princess Candidate 1,000 years ago, a princess of the Magic Realm, who refused the Tiara to save her friends, but lost her friends and her world to destruction as a result.
Yucie stumbles across Cocoloo's drawing, which breaks the judge's spell and unlocks all of her memories. The princess of the Magic Realm chides Yucie for not accepting her fate, but Yucie tells her how important friends are, as they are her true strength, and recalls all the great moments she had with each of them. With her power as the Platinum Princess, her tears shine brightly on the ground, causing a miracle.
All the spirits of the Magic Realm rush to Yucie, who can now hear them. She tells the princess of the Magic Realm that all who died there still love her, and that they have always been with her these 1,000 years, and they never regretted fighting alongside her to the very end. The judge realizes that she had never been alone and that she had made the right choice in trying to save her friends. Yucie's power covers the ground with Sunset Blossoms, and a Crystal Flower emerges from the ground to the hands of the Princess of the Magic Realm. The 6th and final fragment is added to the Eternal Tiara. Together, the former and present Plantinum Princess combine their strength to make Yucie's wish come true. The Magic Realm finally collapses after its last princess, smiling with joy, disappears.
In the end, Yucie and her fellow former candidates are readmitted into the Academy for further study. With the curse lifted, and the Eternal Tiara again fragmented and sealed, they can finally grow up normally and enjoy their mutual friendship at their own pace.