From the Animeraider:
Shirahase Arumu is a shy junior high school trumpeter. He isn’t the popular kid, and he has trouble speaking with other people, but when he has that trumpet he feels on top of the world. One day when playing alone in a park after running away from a social function he spots a girl playing the French Horn, whom he can’t keep up with.
After entering high school, he discovers that this girl is Seryuu Yuriko, a fairly gifted musician who has attempted repeatedly to give it up because her mind enters a sort of fugue state when she plays, and she doesn’t remember what she does when it happens. She’s big and tough and sexy and angry all at once – can Arumu bring her to just enjoy music?