After Akira Ninomiya miraculously survives a brutal attack by an unknown assailant wearing a "monster mask," he becomes fixated on finding the attacker and getting revenge.
Onodera Yuriko sets off for Sweden where her husband, Major General Onodera Makoto, is stationed as a military attache in Stockholm during World War II. Called the "god of intelligence", Makoto is an intelligence officer of the Russian service of the Japanese Army General Staff. Fluent in Russian and German and trusted by the spies of many countries because of his integrity, his office would eventually become the most important Japanese intelligence post in Europe. From the day of her arrival in Stockholm, Yuriko helps her husband's intelligence activities. She encrypts the highly classified information obtained by Makoto and sends it in coded telegrams to the General Staff Headquarters in Japan every day. Husband and wife have jointly undertaken this intelligence work for confidentiality.
Nozomi Takemoto, a reclusive first-year high school student, resides in Omuta with her father, a town that once thrived as a coal mining hub. As her summer break nears its end, Nozomi embarks on a bicycle ride to the town's outskirts and stumbles upon a captivating relic: the remnants of a coal mine turret that once symbolized prosperity. Intrigued by this discovery, she delves into the town's history, scouring the library for old records and materials. Along her journey, she crosses paths with a newspaper reporter assigned to cover the town's post-mining era. Through this summer adventure in an unfamiliar town, Nozomi gradually uncovers the hidden chapters of her family's past, connecting her with the experiences of her father and grandparents.