Sousuke Kaburaya, an impoverished detective, meets Sara, a princess from another world with magical powers. They start living together, and Sara quickly adjusts her life to modern Japan.

When the "Sirens," an alien force with an arsenal far surpassing the limits of current technology, suddenly appeared, a divided humanity stood in complete solidarity for the first time. Four countries—Eagle Union, Royal Navy, Sakura Empire, and Iron Blood—formed Azur Lane, paving the way for the improvement of modern warfare, which led to an initial victory against the common threat. However, this tenuous union was threatened by opposing ideals, dividing the alliance into two. Sakura Empire and Iron Blood broke away and formed 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, afraid of the ocean. Even so, she continues to fight as she believes that it's 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. [Written by MAL Rewrite]

Wisteria is an orphan girl living in a corner of the British Empire at the end of the 19th century. Her life is desolate and bleak–until she encounters Marbas, a powerful but equally lonely immortal being with a furry appearance, hounded by hunters. Together, Wisteria and Marbas roam the Empire–populated by humans and human-like beasts–in search of a place where they can live together in peace.

After inheriting the company from their parents, siblings Yuuhi and Yuamu Oudou have started to work at Ultraterrestrial Trouble Solutions, an organization that hunts aliens. Although they have never encountered aliens before, Yuuhi is desperate to meet one as he believes that an alien has stolen his important item called the "Earthdamar." One day, while searching for extraterrestrial creatures, Yuuhi and Yuamu find a strange spot in the forest where many trees have fallen, forming a scenery similar to a crop circle. As they venture further, they discover a mysterious spaceship and enter it out of curiosity. To their surprise, they come across Yudias Velgear, an alien from the Velgear Star Cluster. As it turns out, Yudias is seeking to learn about Rush Duel—a card game popular among elementary school students—as he believes that it will save his home from a long war. With the siblings agreeing to teach Yudias about Rush Duel, they eventually realize that their first contact with an alien may not be a random coincidence.