"Zankyou no Terror" is an original anime series set in modern-day Japan, revolving around two teenage boys, Nine and Twelve, who identify themselves as "Sphinx." The duo carries out a series of meticulously planned terrorist attacks across Tokyo, leaving cryptic riddles as warnings to the authorities. Their actions are not random; they are driven by a mysterious past tied to a secret government project known as "Athena," which involved human experimentation on children to enhance their cognitive abilities. Nine and Twelve, along with other children, were subjects of this project, and their actions are a form of retribution against the system that exploited them.

The narrative intensifies when Lisa Mishima, a socially isolated high school girl, becomes entangled with Nine and Twelve after a chance encounter. Lisa, struggling with her own personal demons and a neglectful home life, is drawn into their world, becoming an unwilling accomplice. Her involvement adds a layer of emotional complexity to the story, as she grapples with her loyalty to the boys and her fear of the consequences of their actions.

The series also follows Shibazaki, a seasoned but disillusioned detective who is brought out of retirement to investigate the terrorist attacks. Shibazaki's sharp intellect and persistence make him a formidable adversary for Sphinx. As he delves deeper into the case, he uncovers the dark secrets of the Athena project, connecting the dots between the boys' past and their present motives. His investigation becomes a race against time to prevent further destruction while confronting the moral ambiguities of the situation.

The plot unfolds through a series of high-stakes confrontations, psychological battles, and revelations about the characters' pasts. The relationship between Nine and Twelve is central to the story, with Twelve's more carefree and impulsive nature contrasting with Nine's calculated and reserved demeanor. Their bond is tested as the stakes rise, and their ultimate goal—exposing the truth about Athena—comes at a heavy cost.

"Zankyou no Terror" explores themes of identity, justice, and the consequences of systemic corruption. The series maintains a tense, atmospheric tone, blending elements of psychological drama, mystery, and action. The narrative is driven by its characters' motivations and the moral dilemmas they face, creating a compelling and thought-provoking story.