"Gleipnir" is a dark fantasy anime series that revolves around a mysterious phenomenon where individuals gain supernatural abilities by transforming into monstrous forms. The story centers on Shuichi Kagaya, a high school student who discovers he can transform into a furry, dog-like creature with enhanced strength and agility. This transformation is triggered by intense emotions, but it comes with a cost: Shuichi cannot remember how or why he gained this ability.

The narrative takes a dramatic turn when Shuichi encounters Clair Aoki, a sharp-witted and determined girl searching for her missing sister. Clair discovers Shuichi's secret and blackmails him into helping her uncover the truth behind her sister's disappearance. Their investigation leads them to a group of other individuals with similar monstrous transformations, all of whom are connected to a deadly game orchestrated by an enigmatic organization. Participants are forced to hunt and kill each other to collect coins, which are believed to grant a single wish to the last survivor.

As Shuichi and Clair delve deeper into this dangerous world, they uncover unsettling truths about their own pasts and the origins of their powers. Shuichi's transformation is revealed to be linked to a tragic incident involving his childhood friend, Elena Aoki, who is Clair's sister. Elena, now a key figure in the game, possesses her own monstrous form and a complex relationship with Shuichi. The series explores themes of identity, memory, and the moral ambiguity of survival, as the characters grapple with their humanity and the consequences of their actions.

The dynamic between Shuichi and Clair evolves throughout the series, with Clair's manipulative tendencies contrasting with Shuichi's more passive and empathetic nature. Their partnership is fraught with tension, as Clair's relentless pursuit of her goals often puts them in peril. Supporting characters, such as the enigmatic Kaito and the ruthless Honoka, add further complexity to the story, each driven by their own motivations and secrets.

The plot is marked by intense action sequences, psychological drama, and a steadily unraveling mystery. The series balances its darker elements with moments of introspection, as the characters confront their fears and desires. The narrative builds toward a climactic confrontation with the forces behind the game, culminating in revelations that challenge the characters' understanding of their world and themselves.