
Gantz
After trying to rescue a man on the subway tracks, two teens wake up in a room dominated by a mysterious black sphere that sends them to hunt down and kill aliens hiding on Earth.
100 years ago, titans suddenly appeared on Earth. Soon, human civilization veered on collapse due to the titans. Humans then built a giant wall to defend themselves. Within the giant walls, humans lived in peace, but, 100 years later, the giant wall is broken.