Hikaru Amatsuga's days of tranquility come to a sudden end when he rescues Alma, a mechanical arm crying for help. Alma fuses with his jacket, introducing Hikaru to the world of mechanical arms: mysterious, sentient weapons that normally become a part of their users' bodies. Forced to flee for his life, Hikaru is eventually captured by Aki Murasame, a twin mechanical arms user working for ARMS, an organization that protects the machines against unethical treatment. The Kagami Group, a nefarious organization that treats mechanical arms as mere tools, seeks to recover Alma. Hikaru is deemed a failure by ARMS for his lack of natural predisposition to be an mechanical arm wielder, so Aki is assigned to be his bodyguard. But Aki is defeated by Jun Kagami, who cruelly tears out her two mechanical limbs, condemning the girl to a slow death. Unwilling to abandon Aki to her grim fate, Hikaru miraculously awakens his power and neutralizes Kagami. Now a full member of ARMS, Hikaru is determined to unravel the truth about mechanical arms and protect his newfound friends and allies.

Noel longs to be a Seeker like his heroic grandfather, slaying the beasts that emerge from Abysses and exploring far-off lands. Unfortunately, it turns out he's nothing but a measly Talker–a job with no combat skills whatsoever. Undaunted, Noel sets out to establish the strongest clan in all the land, using his silver-tongued Talker skills to unite assassins and heroes under his leadership.

As the Love Live! Competition draws to a close, Kanon Shibuya takes an offer to study abroad and bids farewell to her friends and fellow members of the school idol group Liella!. Although they will live on opposite sides of the world, both Kanon and the group remain motivated to continue their idol activities. Unbeknownst to Kanon, however, her plans are cut short by Wien Margarete's parents, who request that Kanon remain in Japan for another year so that their daughter can learn from her. While Kanon finds herself in awkward limbo, Wien cannot bring herself to join her once fierce rival Liella! and decides to start a group of her own. Not wanting Wien's determination to go to waste, Kanon joins the new group, recruiting her former group member's sister Tomari Onitsuka in the process. As the newfound unit strives to wash off Wien's controversial past, the new Liella! is also wrestling with the struggles of losing their irreplaceable center.

Twelve-year-old prodigy Rafal believes in living rationally, so as to earn praise and respect from society while not being led astray by his emotions. To this end, he publicly states his intention to study theology—the academic field held in highest regard in early 15th century Poland. However, an encounter with a mysterious man upends Rafal's life, sparking an illogical desire to instead pursue his passion for astronomy. Rafal is determined to prove the beauty and rationality of heliocentrism—the theory that the Earth revolves around the Sun. This belief is considered heretical by the powerful Church, which promotes geocentrism—the Sun revolving around the Earth—as the sole truth of the universe. Those whose beliefs do not align with the will of the Church suffer unfathomably gruesome consequences. In pursuit of evidence for a heliocentric model of the universe, Rafal grapples with obtaining precise calculations and building empirical theories. His greatest challenge, however, lies in conducting this research discreetly—lest he wish to meet the same fate as other heretics.