The Prophecy Book is a notebook collaboratively written by Kenji and his friends when they were kids, mostly in their secret hideout. The book predominantly records the group's childish, manga-inspired hero fantasies. These eventually converge into a narrative of events in which the group unite as adults to stop an evil organization bent on destroying the world with biological weapons and giant robots. The Prophecy Book is one of the original documents containing the unique eye/hand symbol designed by Otcho.

Many years after the book was written, the mysterious cult leader known only as 'Friend' orchestrated numerous criminal and terroristic events which eerily followed the narrative recorded in the Prophecy Book. The events of 20th Century Boys which begin in 1997 are largely caused by Kenji becoming aware of this pattern, especially after he draws the connection between the apparent perpetrators and the eye/hand symbol, supposedly known only to Kenji and his childhood friends.

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