A lonely orphaned princess makes the acquaintance of a kindly unicorn whose friendship helps the struggling girl to finally realize what true happiness is all about in this animated children's fantasy from premiere French children's author Grégoire Solotareff and filmmaker Serge Elissalde. Mona (Isild Le Besco) lives a lonely existence in a crumbling castle on the shores of the sea -- where she is cruelly mistreated by a pair of bullying rats. Just as it seems that Mona will never find her place in the world, a coltish unicorn named U (Vahina Giocante) appears before her and claiming that it is her duty to set Mona down the path of happiness. Over the course of the next few years, Mona and U become the very best of friends. When Mona blossoms into a charming preteen and U returns home one day with a playful lizard named Lazare (Guillaume Gallienne), Mona fast becomes friends with both the congenial reptile and his family -- a boisterous group of musicians who reside in a nearby tree. When the rats voice their displeasure at the company of Mona's new guests and the newly empowered princess finally stands her ground against the tyrannical rodents, the once quiet castle soon becomes a place of happiness and merriment. As love begins to blossom between Mona and guitarist Kulka, U ponders an uncertain future now that her selfless mission in life is finally complete.