Pars Era, the year 320—war is on the horizon. As the soldiers of Pars prepare for battle, a traitor within their ranks turns the tides in favor of the enemy kingdom, Lusitania. Arslan, the young Prince of Pars, survives the massacre to discover that his father has gone missing and his life is in grave danger. Unsure who to trust, Arslan escapes the battlefield, evading capture until he’s found by his loyal friend Daryun, the formidable Black Knight of Pars.

Spring in the first year of high school. Kumiko, a member of the brass band in junior high school, visits the high school brass band club with classmates Hazuki and Sapphire. There, she comes across Reina, her former classmate from junior high. Hazuki and Sapphire decide to join the club, but Kumiko can’t make up her mind. She recollects her experience with Reina at a competition in junior high school.