From the introduction:
Welcome to the new world of Kamen Rider! Things aren’t as they used to be, here in the new decade of the second millennium. This world is similar to the one you hail from, but of course there are some differences. This world hearkens back to the origin of the Riders, people with noble hearts who take the powers of evil and use them for justice against the wicked. Human nature plays a stronger role in these stories, however, as many of the monsters are simply people like you and me, who have fallen into the temptation that power so brings. Will you play into this temptation as well? I can't say, nor can I judge, but know that many of the so-called kaijin are people who wished to live without regret, and simply went in over their head.