Ryouhei Arisu is a high school student with no ambitions and just wants to escape from what he feels is a meaningless reality. One night when he is with his two friends Daikichi Karube and Chouta Segawa, they see an abnormally large firework shoot into the sky, an event which signals a permanent change in their lives. Blinded by the incredibly bright explosion, the three find themselves transported to what is known as the Borderland. The only inhabitants of this world are participants of a deadly game where specific tasks must be accomplished if they wish to survive. As the three begin playing to determine how they can get back home, Ryouhei finds himself feeling alive for the very first time in his life. [Written by MAL Rewrite]

Rika Furude and her group of friends live in the small mountain village of Hinamizawa; in June 1983, they welcome transfer student Keiichi Maebara into their ranks, making him the only boy in their group. After school, they have fun playing games and spending each day living their lives to the fullest. Despite this seemingly normal routine, Keiichi begins noticing strange behavior from his friends, who seem to be hiding the town's dark secrets from him. Elsewhere, a certain person watches these increasingly unsettling events unfold and remembers all the times that this, and other similar stories, have played out. Using that knowledge, this person decides to fix these broken worlds. However, when certain variables change, the individual is faced with a horrifying realization: they have no idea what to expect or how to stop the impending tragedy. [Written by MAL Rewrite]

Ryo Saeba (aka the City Hunter), is a private detective in the city of Tokyo. Together with sidekick Kaori Makimura, Ryo is hired by actress Emi Makaze to find her long lost brother. The woman's brother turns out to be "The Professor", a dillusional ex-soldier who's planing to destroy the Shinjuku district of Tokyo. Kaori is assigned to guard Emi, but they both get caught up in the Professor's scheme. The City Hunter must save them, without allowing downtown Tokyo to be destroyed. 

(Source: ANN)

Extra episodes of Killer Seven.