The Place Promised In Our Early Years

In an alternate postwar timeline, Japan is divided into the Union-controlled North and the US-controlled South.
In the summer of 1996, three middle-school students make a promise that they'll cross the border with a self-constructed plane, but their project was abandoned after Sayuri became mysteriously ill. Years later, Hiroki finds out that Sayuri had been in coma since then, and he resolves to wake her up.