This is from the same guy who did Voices of a Distant Star.
It's set in an alternate universe, Japan's split into North and South. Two teenage best friends have two obsessions, a girl they both have a crush on, and building a plane to fly with her to an enormous tower on the horizon in enemy territory. Unfortunatly, the girl goes into a coma and the guys go their separate ways. 3 years later, war between north & south is coming, a link between the girl's coma, the tower, and alternate universes is found, and the boys realize that the only way to wake up the girl is to complete their promise and fly to the tower. But if she wakes up, the tower might destroy the entire world.
The animation's pretty, the story's slow and has little action, and the ending's happier than Voices. The movie was ok, but had the annoying tendancy to occasionally jump from present to 3 years ago and from reality to dreamland. Also, several things weren't explained, like why the enemy would build a giant tower to possibly destroy the world, and why this girl was connected with the tower.
Voices was better, anyway.