Princess Tutu

Ahiru was a regular student at Kinkan Academy. Till one day she sees a new shop that has a pretty tutu in the window and the woman who owns the shop Miss Edel gives Ahiru a pendant. Later she see a boy from school whom she thinks of as a prince named Mytho standing by a lake. Suddenly a monster from the lake is about to attack Mytho. Wishing to save him she suddenly finds herself transformed into Princess Tutu. She returns a piece of his heart to him. The next day Miss Edel tells Ahiru that she must become Princess Tutu and find and return all the missing pieces of a pure Princes's heart. Later she meets Princess Kraehe who like Mytho's friend Fakir does not want Princess Tutu to restore Mytho's heart.

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Summary from another:

Ahiru is a gawky young girl whose passions are ballet and a certain boy who studies it with her. When her clumsiness hinders her performance, she dreams of the miracle that will transform her into a prima ballerina. A spellbound pendant soon answers her prayers, opening an enchanting world of magic and wishes, where Ahiru is the beautiful and graceful Princess Tutu, and she can capture the attention of her beloved Mytho. The two appear destined to be together, but cruel fate has other plans...