In normal bear life, it is frowned upon to make friends with a mouse. But Ernest, a big bear, a clown and musician who lives on the fringes of bear society, nonetheless welcomes little Celestine into his home. She is an orphan and has fled the mouse world down below. These two solitary characters find support and comfort in one another, but in the process, fly in the face of convention, upsetting the established order.
At one level this is a simply beautiful animation about a mouse and bear becoming friends. At another level it is deep social commentary about the challenge of overcoming divisions and prejudice. As an allegory of racial, religious or ideological intolerance, it's moving and profound, bringing to mind the graphic novel Maus.
If you just want to watch an adventure cartoon, it's that too.