From Bakaupdates: Leopold Fieseler, a detective with the Zeon Public Peace Department/Zeon Public Safety Department is charged with investigating terrorist attacks against Zeon government officials led by an anti-Zabi group called Valkyrie. While researching to find out who were the culprits, Leopold gets into trouble when Valkryie supporters within the ZPPD/ZPSD "arrest" him for getting his hands into classified information to help find out who are the culprits for a planned terrorist attack against Gihren Zabi himself.

Suddenly, a fleet of ships with mysterious superweapons called the “Fleet of Fog” appeared all around the world. Humanity didn’t have the technology to fight such a fleet, so they were driven out of the sea... Seventeen years after the battle, for some reason, Gunzou Chihaya and his crewmates aboard the I-401 and the personification (Mental Model) of the I-401, Iona, started fighting the Fleet of Fog...