Planet Earth, Year 2013.
After the horde of monsters came out of the gate, humans figured out that technology doesn't work on them, guns can't penetrate their skins, missles didn't active once targeted at them, and so many other technologies didn't work against the monsters. But humans found that out the hard way, and that the only way to fight of the monsters was to use the powers given to them by the Geno systems and absorb the Geno cores that are inside of the monsters heart, so as to get strong enough to kill monsters with the power of the Geno systems. ...
After a few more years of battling with the monsters, they were finally able to repel the monsters back into the newly called "Geno Gates."
And in the year humans celebrated their victory over the demons and monsters, after the monsters were repeled back into the gates humans set up guards all around the GENO Gates, to make sure no one mistakenly enters the gates or if any monster tries to sneak out into the world.
But soon rather than later, some humans still wanted to get stronger, so they banded together and called theirselves the solo force and so they created a plan to sneak into the Geno Gates, so after a few months of planning they decided to put their plan in action.
And soon they successfully sneaked into the gates.
After entering the gates they were immediately scared shitless by the horrors of the world. A world of mythical beasts that they had never seen in the horde of monsters that attacked the Earth, A blood red sky that was incredibly scary,and even some powerful beasts that people haven't heard of before, they also realized that the Earth's beast horde were nothing compared to all the beasts in this world if even some of them alone attacked the Earth they could have wiped out the entire human race by now, and with such horrors right in front of them, some even fainted right on the spot.
But they were still determined to get stronger than any other human ever in this new world of demons and monsters. Then they started their adventure into this new world.