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.

After a momentous New Year's vacation and with Valentine's Day approaching, Sawako Kuronuma is beginning to get along with her classmates. However, now that Sawako has realized her romantic feelings for the popular Shouta Kazehaya, she grows hesitant toward giving him obligatory chocolates and decides to not give him any. In turn, Kazehaya, who likes Sawako, feels a distance between them. As February ends and April arrives, the second year of high school begins for Sawako. Luckily, she ends up in the same class as her friends Ayane Yano and Chizuru Yoshida, along with Kazehaya and his friend Ryuu Sanada, in addition to the newcomer named Kento Miura. When Kento develops an interest in Sawako, Sawako and Kazehaya's feelings for each other are put to the test.

In the mysterious future, crystalline organisms called Gems inhabit a world that has been destroyed by six meteors. Each Gem is assigned a role in order to fight against the Lunarians, a species who attacks them in order to shatter their bodies and use them as decorations. Phosphophyllite, also known as Phos, is a young and fragile Gem who dreams of helping their friends in the war effort. Instead, they are told to compile an encyclopedia because of their delicate condition. After begrudgingly embarking on this task, Phos meets Cinnabar, an intelligent gem who has been relegated to patrolling the isolated island at night because of the corrosive poison their body creates. After seeing how unhappy Cinnabar is, Phos decides to find a role that both of the rejected Gems can enjoy. Houseki no Kuni follows Phos' efforts to be useful and protect their fellow Gems. [Written by MAL Rewrite]

Even after spending a considerable amount of time with Takagi, Nishikata is still struggling to find a perfect plan to defeat the expert teaser. A battle of wits, a contest of physical prowess, a test of courage—any strategy he employs to expose her weaknesses is to no avail. On the contrary, Nishikata's pitiful attempts only reveal more of his own flaws, which Takagi takes advantage of to become increasingly daring in her teasing attempts. To make things worse for Nishikata, rumors about him and Takagi may have spread in class due to the frequent interactions between them. However, the optimistic Nishikata believes that wisdom comes with age and that as the days go by, his experience with her constant teasing will eventually bear fruit, leading him to the awaited moment of victory. Thus, Nishikata continues to strive for the seemingly impossible—to outsmart Takagi and make her blush with embarrassment. [Written by MAL Rewrite]