Clavier is a grumpy little rat who never quite fits in with his lively siblings. When he sneaks a spoonful of sticky honey into his little sister's mitten, his mother decides he's staying home from the Christmas fair. From the old church bell tower, he watches his family scurry through the snow, the scent of warm gingerbread and cinnamon making his whiskers twitch. He has no idea that missing the fair will lead him to the biggest discovery of his life.

That afternoon a black car rumbles into the church parking lot and out steps a mysterious old man with a briefcase. Clavier's ears perk up. Soon, strange sounds fill the air. Deep rumbles like a runaway train, curious clicks and clatters, and something else . . . a melody, drifting through the church vents. Where is it coming from? And why is the old man sneaking through the attic at midnight?

Determined to solve the mystery, Clavier embarks on a whisker-raising adventure that leads him to a long-lost musical secret hidden in plain sight. But when the old man lands in trouble on Christmas Eve, Clavier is the only one who can save the holiday surprise.

Can a tiny rat with big doubts find the courage to play his part?