Around the time the damages caused by the First Sino-Japanese War were beginning to heal, Mihagi Sonoo had graduated from a regular school. Unable to find employment, she found herself in poverty.
Then, an opportunity had arisen. Not far from her home was the Yokosuka Military Base, and they were looking to hire artists. Having learned of this, Sonoo headed for a job interview with the drawings she had made. However, while she was lost in the building, she accidentally witnessed the military band working as spies (they were in a meeting), and to top it off, she got caught doing so.
With the military band not wanting their secret to get leaked and with Sonoo looking for work, they came to an agreement: she became the military band's "artist-cum-spy apprentice" and became a member.
For a while, she was tasked with drawing posters and did some general work on the side. One day, her superior officer, Iyobe, summoned and ordered her to join the other band members to engage in actual spy missions.
And that was how the first mission of her life began.
[Translation of the official website story summary.]