In the last months of the Second World War, the Empire of Japan receives a final gift from the collapsing Nazi Germany: the I-507, a highly-advanced submarine equipped with experimental technology.
The mission, as revealed by the grim Chief of Staff Asakura (Shinichi Tsutsumi) following the Atomic bombings of Hiroshima and Nagasaki, is to intercept U.S. ships transporting a third nuclear weapon to Tinian Island, the principal base from which American B-29s Bombers are striking the Japanese home Islands. The man charged with the mission is Commander Masami (Yakusho Koji) - a brilliant destroyer of enemy ships relieved of his command when he opposed the navy's increasing reliance on suicide tactics. Given a last chance to redeem himself, he is burning with zeal, but is ignorant of the various secrets the I-507 carries on board.
Once at sea, Lt. Takasu (Ken Ishiguro), the owlish technician in charge of the imaging system, refuses to tell Masami what it is or how it works. Masami also discovers that two crew members belong to the "kaiten" suicide corps. He has no idea why they are there -- and neither, for the moment, do they.
Meanwhile, the U.S. Navy is tracking the I-507 with more than usual interest. What, Masami wonders, is going on here? Enough to say that a sweet-voiced teenage girl (Yu Kashi) is part of the master plan and that one of the minisub pilots (Satoshi Tsumabuki) becomes her protector -- and something more.
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