An enormous cylindrical alien spaceship, which people call "the Bell" (釣り鐘, tsurigane), descends upon Earth one day. Multinational military forces attack it, but the Bell's defenses easily vaporize all human weaponry. The story takes place one year afterwards. The Bell, which seems to have no living crew, has begun sending out robotic probes to explore and analyze human civilization and all of the aspects of it. For most of the series, the Bell hovers quietly above the ocean, several miles from Tokyo. It had soon become apparent that the Bell's defenses only destroy whatever attacks it first. As such, most people have now adapted to the Bell's presence on Earth and are once again going about with their everyday lives, just making sure to stay out of the way of the Bell's probes when they pass by. A lot of people, however, due to a message sent by the Bell, believe the Bell may eventually attack and conquer Earth, and are working to figure out a way to stop it.