In their last year of high school, two girls in the brass band club perform a song inspired by a fairy tale that parallels their friendship.

Kumiko and her teammates have won the Kyoto regional tournament. From now on, life will be getting a lot more hectic. With the national tournament fast approaching, Kumiko's band is doubling down on practice and discipline, leaving little time for sleep or other recreational activities. The redoubled practice schedule brings Kumiko and Reina closer together, but elsewhere in the band, it seems like drama threatens to tear the group's unity apart. The ghost of the prior year's mass band exodus still haunts many of Kumiko's classmates, and with former band member Nozomi Masumi now asking to return, it seems like the grudges of the past will soon be impossible to ignore.

这个世界上存在着异形之物，其姿态千变万化，即为“妖梦”，以除去妖梦为业的，即为异界士。本作主人公神原秋人（KENN 配音），看起来是个普通的高二男生，实际上是由人类和妖梦结合而诞生的“半妖”。拥有不死之身的他，于某个黄昏，在校园天台遇见一名想要跳楼的少女栗山未来（种田梨纱 配音），出乎意料的是，少女居然对他发起了攻击。原来，看起来是个普通新生的眼镜美少女，竟是一名能够操纵血的异界士。 拥有特殊能力的栗山未来，引起了异界士名门望族名濑家的注意。与秋人同为文艺部的名濑美月（茅原实里 配音）即是名濑家的次女，她嘱咐秋人远离栗山。肆虐的妖梦，孤独行走于世的异界士，以及罕有的半妖，在境界的彼方展开一个奇异的幻想故事。