In present day Japan, during what should have been a routine training exercise, Captain Umeza and the battleship Mirai are suddenly hit by a storm, sending them back in time to the middle of World War 2. Armed with state-of-the-art technology, the Mirai could alter the course of history; but for its crew, the situation presents a moral dilemma: do they have the right to meddle with history, even to stop the most dreadful tragedies of the war, when even the smallest acts could have unforeseen consequences?