Life isn't easy when you're a transfer student - just ask Minato Arisato, Gekkoukan High School's newest transfer student. Strangers and unfamiliar scenery aside, Minato also has to contend with the fact that his school transforms into a Shadow-filled tower called Tartarus every night during the Dark Hour Minato and two of his new friends - Yukari and Junpei - set out to investigate Tartarus to see what they can learn about this phenomenon, but can three high school students with little to no combat experience possibly survive the tower's horrors?