The story begins with the protagonist, a black rabbit named Kuro, reflecting on a past life and a deep regret, though the specifics remain unclear. She lives a peaceful life in a forest, caring for young rabbits in exchange for food. This idyllic existence is shattered when she awakens to find the forest completely gone, replaced by barren land. Overwhelmed by fear and loneliness, she encounters a powerful, golden-eyed man with fiery hair, who is revealed to be a Prince. He is surprised to find a rabbit in this desolate location and, despite initial apprehension from Kuro, decides to take her with him.
The Prince, later identified as Virinji, is accompanied by his stoic butler, Al. Al is initially resistant to the idea of bringing another pet into their already crowded household, which Al describes as a "mini zoo" with thirty-seven animals. However, the Prince is captivated by Kuro's unusual calm and direct gaze, a stark contrast to the fear or aggression he usually elicits from small animals. He insists on taking her, leading to a compromise where Al agrees if the Prince relinquishes three existing pets. Kuro is then brought to the Prince's opulent residence.
Upon arrival, Kuro is introduced to the household staff, including Mara, a kind maid who prepares a comfortable basket for her. Kuro's presence immediately charms the staff, who find her incredibly cute. The Prince, Virinji, is clearly smitten with Kuro, finding her every action endearing. He is also revealed to be the ruler of the country and is currently overwhelmed with work due to recent purges of corrupt nobles.
Kuro begins to ponder the cause of the forest's destruction and the fate of her rabbit companions. She suspects magic, a rampaging monster, or a natural disaster. She hopes to gain information from Virinji, Al, or Mara. Her newfound life in the castle is filled with overwhelming experiences, including being introduced to Virinji's ten wolf-like dogs, which initially terrify her. She also witnesses Virinji's intense work ethic and his unusual reactions to her presence.
A significant turning point occurs when Kuro, through her actions, demonstrates an ability to understand human speech and discern good from evil. This is revealed when a maid attempts to poison Kuro's milk, and Kuro's reaction, along with a magical detection tool provided by Mara, exposes the maid's treachery. Virinji, realizing Kuro's intelligence, begins to suspect she is more than just a rabbit.
Further revelations unfold when Kuro is taken to meet the Knight Commander, who identifies her as a "magic user" and potentially a "Sacred Beast." The Commander explains the world's magic system, categorizing creatures based on their magical abilities and attributes. He reveals that Kuro possesses multiple attributes (earth, water, light, darkness) and an immense amount of magic, placing her close to a "Mythical Beast" and already ranking her as an A-class individual, despite her current lack of control. This discovery is a shock to Virinji, who had no idea of Kuro's true nature.
Kuro's ability to understand human language is confirmed, and a magical translation device is commissioned to bridge the communication gap. She also learns that Virinji's often peculiar behavior and physical reactions are related to his own magic and possibly a past trauma. The story progresses with Kuro actively participating in castle affairs, using her discerning nature to help identify untrustworthy individuals during interviews. She becomes a beloved figure in the castle, known as the "Black Sacred Beast of Healing," bringing a sense of calm and joy to the inhabitants, particularly Virinji. The narrative concludes with Kuro anticipating the completion of her translation device, eager to communicate more fully and understand her place in this new world.
