In August 20XX, a portal to a parallel world known as the "Gate", suddenly appeared in Ginza, Tokyo where monsters and human warriors wearing ancient armor poured out of the portal, killing everyone in their way and trying to claim the area as their own. This would later be known as the "The Ginza Incident". Immediately after the attack, the Japanese Ground-Self Defense Forces launches a counter attack, repelling and pushing the invaders back to the Gate and reclaiming Ginza.
Hoping to establish diplomatic relationship with the people beyond the Gate and capture the ones responsible for the massacre of Ginza, the JSDF, under orders by the Prime Minister, takes over and enters the Gate where they dispatch the Third Reconnaissance Team to scout the "Special Region" led by 33 year old First Lieutenant Youji Itami. Itami was there when the Ginza incident happened and, luckily for him, exploring the Special Region is a dream come true since the Lieutenant is an Otaku. While the 3rd Recon Team explores the Special Region, back on Earth, the major powers take a special interest with the Gate...
