Aoki Hagane no Arpeggio: Ars Nova Cadenza (Blu-ray release 2016-3-25) is the second movie following a TV anime series and a movie, which are prerequisites. As one may imagine, this movie introduces more cute girls who are actually sci-fi battleships with awesome powers. It provides more of the same submarine / battleship battles, but what makes this series so good is that each is won differently, from a rich story that is not just about fighting the bad guys. To my surprise it actually provides a satisfying conclusion to the story.
Note: After the credits there is an extra scene, and some people interpret it as suggesting what is implied in the climax is reversed somehow.