When the "Sirens," an alien force possessing an arsenal that far exceeds the limits of current technology, suddenly appeared, a divided humanity stood in complete solidarity for the first time. Four nations—the Eagle Union, Royal Navy, Sakura Empire, and Iron Blood—formed Azur Lane, paving the way for the advancement of modern warfare, which led to an initial victory against the common threat. However, this fragile union was threatened by opposing ideals, splitting the alliance into two. The Sakura Empire and Iron Blood broke away to form the Red Axis, and humanity became fragmented once again. As a seasoned and experienced fighter, the "Grey Ghost" Enterprise shoulders Azur Lane's hope for ending the war. But behind her stoic persona hides a frail girl who is afraid of the ocean. Even so, she continues to fight, believing that it is the only purpose for her existence. Meanwhile, Javelin, Laffey, and Unicorn—three ships from the union—stumble upon Ayanami, a spy from the Red Axis. Strange as it may seem, they try to befriend her, but as enemies, their efforts are for naught. Still, they persevere in hopes of succeeding one day. Amidst the neverending conflict within humankind, the keys that could unite a fragmented race might exist: a soldier coming to terms with her mysterious personality and camaraderie between those with different ideals.