"C³" is a 13-episode anime series based on a light novel series by Minase Hazuki. The narrative centers around Haruaki Yachi, a high school student who leads an ordinary life until he receives a mysterious black cube from his father. The cube, named Fear, is revealed to be a cursed tool with a humanoid form. Fear, who takes the appearance of a silver-haired girl, is cursed to inflict pain on anyone who touches her. Despite her dangerous nature, Haruaki decides to protect her and help her break free from her curse.

As the story progresses, Haruaki and Fear encounter other cursed tools, each with their own unique abilities and tragic pasts. These tools, known as "Worries," often manifest as humanoid beings and are driven by their curses to cause harm. Haruaki, alongside Fear and his childhood friend Konoha Muramasa, who is also a cursed tool, works to understand and resolve the conflicts surrounding these objects. Konoha, who wields a sword form, serves as a protector and ally, balancing her loyalty to Haruaki with her own struggles as a cursed entity.

The series explores themes of redemption, the nature of curses, and the bonds formed between humans and cursed tools. Fear's journey is central to the narrative, as she grapples with her identity and the guilt of her past actions while seeking a way to overcome her curse. The dynamic between Haruaki, Fear, and Konoha forms the emotional core of the story, as they navigate the challenges posed by other cursed tools and the organizations that seek to control or destroy them.

The plot is driven by episodic encounters with various cursed tools, each presenting moral dilemmas and battles that test the characters' resolve. These encounters gradually reveal the larger world of cursed tools and the organizations that manipulate them, such as the Knights Dominion, a group dedicated to eradicating cursed objects. The series balances action, humor, and emotional moments, creating a narrative that delves into the complexities of its supernatural premise while maintaining a focus on character development and relationships.