"Brynhildr in the Darkness" is a dark science fiction anime series based on the manga by Lynn Okamoto. The story centers on Murakami Ryota, a high school student who has been haunted by the death of his childhood friend, Kuroneko, during a tragic accident years earlier. Ryota, an amateur astronomer, has spent his life searching for extraterrestrial life, partly as a way to cope with his guilt and grief. His life takes a dramatic turn when a new transfer student, Kuroha Neko, arrives at his school. Neko bears an uncanny resemblance to Kuroneko, sparking Ryota's curiosity and suspicion.

As Ryota grows closer to Neko, he discovers that she is part of a group of genetically engineered girls known as "witches," who possess supernatural abilities but are hunted by a secretive organization. These witches are the result of unethical experiments, and their powers come at a cost: they suffer from a condition that shortens their lifespans. Neko, along with other witches like Kazumi, Kana, and Kotori, is on the run from the organization, which seeks to capture and eliminate them to cover up their existence.

Ryota becomes entangled in their struggle, driven by his desire to protect Neko and uncover the truth about her connection to Kuroneko. The series delves into themes of survival, morality, and the consequences of scientific experimentation, as the characters navigate a world where their lives are constantly at risk. The narrative is marked by intense action sequences, emotional confrontations, and a gradual unraveling of the mysteries surrounding the witches and the organization pursuing them.

The plot explores the relationships between the characters, particularly Ryota's growing bond with Neko and the other witches, as they form a makeshift family in the face of adversity. The series balances moments of tenderness with darker, more violent elements, reflecting the harsh realities of their situation. As the story progresses, Ryota and the witches uncover deeper secrets about their origins and the true nature of the experiments that created them, leading to a climactic confrontation with the organization.

"Brynhildr in the Darkness" is characterized by its blend of science fiction, psychological drama, and action, with a focus on the ethical dilemmas posed by the exploitation of the witches and the lengths to which individuals will go to protect those they care about. The series maintains a tense and somber tone throughout, punctuated by moments of hope and resilience as the characters fight for their survival and a chance at a better future.