Traveling Through the Ultra Universe Just to Protect

An accident caused the protagonist's universe to be discovered by monsters from other universes. The growing darkness in the sky meant that this peaceful universe began to gradually merge into the Ultra Universe. There is no Ultraman, and there is no cross-era technology. It seems that being discovered and destroyed by monsters is the end of this universe. In the battle between the air force and monsters in this world, the protagonist accidentally arrives in the Ultra Universe with the fighter jet... It seems that the only one who can save the world is the protagonist who has traveled across the world. This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.