While following the lives of these three, the viewer learns that Japan is split in two in this world, divided into North (associated with the Soviet Union) and South (occupied by the United States). The tower is in the North, across the border from where the three friends are building their plane. Help with the project comes from a man named Okabe, who the boys know as a factory boss but is actually part of a underground liberation movement dedicated to reuniting Japan. A war develops between the two sides over the tower, and eventually Hiroki flies their restored plane to destroy it with a missile provided by the liberation group.
The reason the tower is such a flashpoint is that it is reaching out to parallel realities, and changing the world to match them. These other realities also make their presence felt through people's dreams, particularly those of Sayuri. When she disappears, we learn that she's fallen asleep, trapped in those dreams. For years, the effect is limited to a small zone around the tower while she remains asleep. As she begins to wake, the effect begins to widen. With Takuya's help, Hiroki takes her along on his flight to destroy the tower, completing her recovery.
That's a whole lot of different threads to follow, and the movie suffers a bit from lack of focus. It takes some work to keep track of everything that's going on. It's all interesting, but I think it could have been just as impactful without including quite so many different components. Either the divided-country or the parallel reality incursion aspect would have served as the large-scale conflict, for instance. Including both seems unnecessarily complex and introduces confusion, and a lot of questions on both fronts remain unresolved in the end.
Despite the extra complexity, The Place Promised in Our Early Days is at its heart a story of friendship and love. That aspect works well, though it could have been explored more fully. There wasn't really time, since so much is spent trying to explain the war and parallel universes. Considering those limitations, the characters are developed nicely and their relationships are clear. Hiroki's struggles when Sayuri disappears, his estrangement and eventual reconciliation with Takuya, and the pair's efforts to help Sayuri recover are all well executed.
At the personal level, The Place Promised in Our Early Days is a satisfying story, well worth watching. Just don't try to hard to understand everything going on around the central characters.
