Running into your new stepsister in the bathroom is not the best way to make a good first impression, which Basara Toujou learns the hard way. When his father suddenly brings home two beautiful girls and introduces them as his new siblings, he has no choice but to accept into his family the Naruse sisters: busty redhead Mio and petite silver-haired Maria. But when these seemingly normal girls reveal themselves as demons—Mio the former Demon Lord's only daughter and Maria her trusted succubus servant—Basara is forced to reveal himself as a former member of a clan of Heroes, sworn enemies of the demons. However, having begun to care for his new sisters, Basara instead decides to protect them with his powers and forms a master-servant contract with Mio to keep watch over her. With the Heroes observing his every move and the constant threat of hostile demons, Basara has to do the impossible to protect his new family members. Moreover, the protector himself is hiding his own dark secret that still haunts him to this day...

Guts, a man who will one day be known as the Black Swordsman, is a young traveling mercenary characterized by the large greatsword he carries. He accepts jobs that offer the most money, but he never stays with one group for long—until he encounters the Band of the Falcon. Ambushed after completing a job, Guts crushes many of its members in combat. Griffith, The Band of the Falcon's leader and founder, takes an interest in Guts and duels him. While the others are no match for Guts, Griffith defeats him in one blow. Incapacitated and taken into the Band of the Falcon's camp to recover, Guts wakes up two days later. He confronts Griffith, and the two duel yet again, only this time with a condition: Guts will join the Band of the Falcon if he loses. Due to his fresh injuries, Guts loses the fight and is inducted by Griffith. In three years' time, Guts has become one of the Band of the Falcon's commanders. On the battlefield, his combat prowess is second only to Griffith as he takes on large groups of enemies all on his own. With Guts' immense strength and Griffith's leadership, the Band of the Falcon dominate every battle they partake in. But something menacing lurks in the shadows, threatening to change Guts' life forever.

Katsuhira Agata is a quiet and reserved teenage boy whose sense of pain has all but vanished. His friend, Chidori Takashiro, can only faintly remember the days before Katsuhira had undergone this profound change. Now, his muffled and complacent demeanor make Katsuhira a constant target for bullies, who exploit him for egregious sums of money. But their fists only just manage to make him blink, as even emotions are far from his grasp. However, one day Katsuhira, Chidori, and four other teenagers are abducted and forced to join the Kizuna System as official Kiznaivers. Those taking part are connected through pain: if one member is injured, the others will feel an equal amount of agony. These individuals must become the lab rats and scapegoats of an incomplete system designed with world peace in mind. With their fates literally intertwined, the Kiznaivers must expose their true selves to each other, or risk failing much more than just the Kizuna System.

Apparitions, oddities, and gods continue to manifest around Koyomi Araragi and his close-knit group of friends: Tsubasa Hanekawa, the group's modest genius; Shinobu Oshino, the resident doughnut-loving vampire; athletic deviant Suruga Kanbaru; bite-happy spirit Mayoi Hachikuji; Koyomi's cute admirer Nadeko Sengoku; and Hitagi Senjougahara, Koyomi's eclectic girlfriend. A new semester has begun and with graduation looming over Koyomi, he must quickly decide the paths he will walk, as well as the relationships he will form and friends that he will save. But as strange events begin to unfold, Koyomi is nowhere to be found, and a vicious tiger apparition has appeared in his absence. Hanekawa has become its target, and she quickly finds she must fend for herself.