Kei Kurono is a self-centered, very indifferent high school student who frequently has a negative attitude of other people. When he sees a drunken man fall over the tracks on a railway platform one day and no one wants to rescue him, including Kei himself, he realizes that his attitude is justified. Masaru Katou, an old classmate from when they attended elementary school with, tries to help and almost coerces Kei into doing so when he asks them to, with grave consequences: as soon as they manage to bring the inebriated onto the platform, they are run over by a train and crushed to death.
At least that's how it appears to the other bystanders, but Kei and Masaru discover themselves in a chamber, where they're not by themselves: In addition to the menacing black sphere, there are other individuals here who essentially all share the same trait: They have all awoken from their deaths in this space. The sphere speaks up before they can fully comprehend what is happening, giving everyone in the room the assignment of eradicating specific extraterrestrial beings within a set amount of time. While the opposition's fighters will nearly vanish like flies, the situation is really just a game for the sphere, in which there will never be a shortage of new characters.
