Japan, Okinawa Prefecture, Okinawa City, Koza District. Saya is a senior high school student, and with her best friend and classmate, Kaori Kinjo, she is a member of the track and field team at school. She lost her memories of her life beyond that of the past year, but she is living peacefully with her family. His father, George Miyagusuku, runs a pub called "Omoro." She also has two brothers, Kai, hot tempered but actually kind and committed to his family, and Riku, who is good natured and loves to read. But Saya's unruffled life is about to change: when she accidentally meets a handsome cellist, Hagi, it is a sign that the wheel of fate has begun to turn...

Saya and her team infiltrate the Yanbaru laboratory. David had learned that the U.S. military is conducting experiments underground to build artificial Chiroptera. There he discovers a mysterious keyword called "Delta 69". Then, a Chiropteran swoops down on Saya's party underground, where they can take no refuge. In order to escape from their subterranean trap, they will have to defeat the creature. Who can save Saya and her party from this desperate predicament?