One day, six towers appear on Earth. Each one stands hundreds of stories tall, and are indestructible to all attacks. At the base of each tower is a door, welcoming in all who dare open it.
All sorts of magical monsters can be found inside the towers. Even magic and superhuman strength can be acquired. Treasure, too. The problem? The towers aren't a friendly place. Even worse, the towers sometimes release waves of monsters to attack Earth.
In the year 2083, there are only a hundred survivors. All of them possess strength beyond normal. They challenge the final floor without regard for their lives. By some miracle, they manage to beat the final floor. Alas, there is only one survivor.
As a reward for clearing the tower, he is granted one wish. He decides to go back in time, to before the towers arrived. His goal? To prevent humanity from being wiped out.