In the early 20th century, the discovery of the substance Cavorite allowed the production of advanced military technology and steered the country toward conflict. London is now divided by a wall, and the Kingdom and the Commonwealth of Albion battle a silent war where espionage is the only weapon that can destabilize the enemy. A group of girls from the prestigious Queen's Mayfaire school work as undercover spies for the Commonwealth. Led by Dorothy, an experienced driver with a striking personality, their group includes the talents of Ange le Carré, a cold-blooded liar and expert sharpshooter; Chise, a proficient samurai; and Beatrice, a voice-mimicking specialist. They use their unique individual skills for the Commonwealth to survive in a dark world filled with conspiracy, mystery, and infiltration. In the shadow of the war, they have only one goal in mind: completing their mission.

There exist few humans in this world with the ability to manipulate their souls to form powerful weapons. Dubbed Blazers, these people study and train at the prestigious Hagun Academy to become Mage-Knights; among the students is so-called failure Ikki Kurogane, the sole F-rated Blazer. However, when the worst student in the academy sees Stella Vermillion, an A-ranked Blazer who also happens to be a princess, naked, she challenges him to a duel with dire stakes—the loser becomes the slave of the winner. There's no possible way that Stella can lose, right? As he tries to prove his strength to a world that believes him to be the weakest, Ikki gains new friends, wisdom, and experience.

In the kingdom of Rimgarde, two mysterious giant robots begin a duel that erases nearly all human life before they disappear. Twelve years later, the young empress of Enastoria, Yuinshiel Asteria, works tirelessly to maintain both her authority and the support of her citizens. But the peace of Enastoria is threatened when survivors of Rimgarde arrive. Armed with giant robots called Regalia Gear, they look for Yuinshiel's adopted sister, Rena. The latter is forced to reveal herself as not just a Regalia Gear wielder, but a real Regalia—a human capable of transforming into a giant robot—and the culprit behind the fall of Rimgarde. Suddenly caught up in an international dispute, Yuinshiel must preserve the security of her country while also protecting Rena from those who wish to abuse her powers. However, by fighting for her sister, Yuinshiel risks triggering the same calamity that once destroyed Rimgarde. Amid such a precarious situation, the two girls push their bond to its limits as they fight not only for themselves but for the world's safety as well.

When Subaru Natsuki leaves the convenience store, the last thing he expects is to be wrenched from his everyday life and dropped into a fantasy world. Things are not looking good for the bewildered teenager; however, not long after his arrival, he is attacked by some thugs. Armed with only a bag of groceries and a now useless cell phone, he is quickly beaten to a pulp. Fortunately, a mysterious beauty named Satella, in hot pursuit after the one who stole her insignia, happens upon Subaru and saves him. In order to thank the honest and kindhearted girl, Subaru offers to help in her search, and later that night, he even finds the whereabouts of that which she seeks. But unbeknownst to them, a much darker force stalks the pair from the shadows, and just minutes after locating the insignia, Subaru and Satella are brutally murdered. However, Subaru immediately reawakens to a familiar scene—confronted by the same group of thugs, meeting Satella all over again—the enigma deepens as history inexplicably repeats itself.