Knowing that he is risking his life, Kei Kurono makes the uncharacteristic decision to jump onto train tracks to save a homeless man from death, only to perish in the process. But when he comes to, Kei finds himself in a room with several other recently deceased people, as well as a large, mysterious black ball. While the people in the room are trying to make sense of their situation, the ball suddenly opens, revealing a cache of weapons and strange black suits. The ball then instructs everyone that they have to hunt down an alien and teleports them to an unknown location. With very little information to work off of, Kei teams up with his childhood friend Masaru Katou and an unfamiliar woman, and the trio explore their new surroundings. It quickly becomes apparent, however, that they have stumbled into a dark and dangerous world where anything could happen to them.