KanColle: The Movie (Blu-ray release 2017-8-30) is the sequel to the 2015 anime series based on a popular browser game. The TV anime was not so great. Other than showing the cute fleet girls (battleships literally in form of girls), it had nothing noteworthy. I do not think it is essential to watch the TV before watching this (I forgot most of it anyway).
One of the fleet girls was sunk in the TV series. In the movie she mysteriously shows up again but remembers very little. A mysterious sound is heard from a real place known as the Ironbottom Sound. The sea also turns crimson, which corrode the gear of the fleet girls. The fleet girls set out to investigate these mysteries and to combat their eternal enemy.
There is a surprisingly dark revelation of the origin of the enemy and their relationship with the fleet girls. It adds much needed depth to the otherwise shallow show. Considering the never ending nature of the game, it actually makes sense from my perspective. At least it is better than the TV series. Note 1: there is an extra scene at the end. Note 2: The Director’s Cut contains 3 minutes of extra scenes.