THE FOG ROLLS IN Chihaya Gunzou, captain of the sentient submarine known as I-401, is renowned for his unorthodox tactics. His unpredictability is a strength when it's just him and his crew against the world, but with two massive Fog battleships attacking their home port, Yokosuka, the I-401 has to team up with a human-built submarine to try to take them down—without letting the Fog ships anticipate their next move! The good news is that the other sub is a "White Whale," representing the most cutting-edge technology Japan can produce. What's more, Gunzou has an invaluable advantage: a former crew member is temporarily serving as part of the White Whale's crew, and she knows how Gunzou thinks. Meanwhile, the Fog battleships Kirishima and Haruna are determined to be the ones to capture the renegade I-401, and they have a plan that no one can see coming.