The movie itself is really set between 1 and 2, so it doesn't really advance the plot line. Also it's more a detective and monster film than a straight sci-fi mecha film. It's kinda nice to see some of the side characters get their time in the spotlight. However, the plotline behind it, while it was well done, was kinda ruined by one one character, the lady researcher. Not to be rude, but she made it really easy to hate her guts. as the story goes, she lost her husband in an accident, and later lost her daughter to cancer; however, she used samples of her daughter and the cancer that killed her in creating the monster. Mother of the Year material, huh?
I won't give away too much about it, needless to say it was nice to watch, but it's not exactly necessary to the whole story to see it.