Mona lives in a ruined castle by the sea. She is a princess but lives a life of misery, mistreated by an odious pair of rats. But one day an adorable little unicorn appears and declares that she is there to assure her happiness.

Mona lives in a ruined castle by the sea. She is a princess but lives a life of misery, mistreated by an odious pair of rats. But one day an adorable little unicorn U appears and declares that she is there to help her through life and to assure her happiness. The two become inseparable. One day U brings home a charming and funny Lizard, Lazare. Scandalized by this intrusion to their home, the rats demand the newcomer leaves. In the surrounding woods they meet Lazare's family, the Wewes, a family of musicians living in a tree. Here life couldn't be more different than in the castle, full of pleasure, music and merrymaking. Mona discovers two new things: the wonder of music and a blossoming attraction to the young guitarist Kulka. However U suddenly becomes aware of a worrying dilemma. What will happen to her if Mona finds the happiness which is the sole reason for her existence...?