OK, a more detailed summary on 20th Century Boys. First, I really love this manga. It is basically a tale of a fight against oppression. Not a very original concept, but this manga delivers it beautifully. The story starts with a group of 6th graders who, in the summers of 1969-71, develop a plan for world domination as part of their games. 30 years later, one of them, Kenji, realizes that someone is actually using that plan "The Book of prophecy" to attack the world. He and his friends band together to stop the cult leader known only as "Friend". What he doesn't know is that "Friend" has developed the "New Book of Prophecy" to follow through to its ultimate conclusion the half-made plans of the children.
The storytelling style is also somewhat unusual and very attractive. The chapters continuously alternate, one chapter telling what happened in the summers of '69-71, and the next talking of the period around 2000, with flashes of what will happen in 2014. Later, as the story progresses, the 2014 period becomes more important, as that is when the final confrontation is due to take place.
Note: the final volumes of this manga goes under the name of 21st Century Boys.