When the Earth's oceans are attacked by a mysterious alien force called the Sirens, four major nations, Eagle Union, the Royal Navy, the Sakura Empire and the Iron Blood form the "Azur Lane" military alliance to combat the Sirens using shipgirls, girls fused with naval warships. Eventually, the Sirens are defeated, but a rift forms between the factions, with the Eagle Union and Royal Navy coming in opposition against the Sakura Empire and Iron Blood. Reinforcements are sent to an Azur Lane base near the Sakura Empire, where Cleveland and Prince of Wales greet the arrival of Illustrious and Unicorn...