Gantz

Kurono Kei, a high school freshman, put on airs and thought only about girls. One day, on his way home, he comes across a man who accidentally fell from the train platform. He didn't care for other people, and at first he enjoyed watching this man that was about to die. However, Katou Masaru, who was a classmate in his elementary days, suddenly gets onto the train tracks from the train platform to help the man. Being asked for help, Kei reluctantly began to help him. But when they got the man onto the platform, the train came. They must have been killed by the train, but they were transferred into a room of a condominium in a moment. There, they found a black sphere and people who are supposed to be dead like them. They couldn't get out of the room, and they couldn't even touch the wall. What is the room? What is this ball?