I didn;t mind the flitting of "main" human characters as much. There were too many of them, and some of them were very wooden, but keeping the father as the link during the 15 year gap was a good idea to tie it together.

I must say that the lead military man must've been exceptionally well trained to be able to do everything that he did.

Watanbe just seemed to be gawking/in awe for most of the film and he had nothing but gut instinct to base his theories on!