the first one is there to introduce the characters, their stories, to make them likeable (or not likeable depending of your taste).. once you do the main story of the rich girl and the childhood friend, you unlock the second muv-luv... wich is REALLY different from the one, and you don't get why... until you play the third one where everything is resolve, if it can be called resolved..I still have no idea what muv luv is about. I only played the first one and it was about marrying a rich girl who hated being a corporate daughter.
Muv-Luv trilogy is one of the best visual novel i had the opportunity to play, though you have to put your time into it