"Angel Heart" is a spin-off of the popular manga and anime series "City Hunter," created by Tsukasa Hojo. The story is set in an alternate universe where the characters and settings share similarities with "City Hunter" but diverge in key aspects. The narrative centers on a young woman named Xiang-Ying, also known as Glass Heart, a highly skilled assassin raised by a shadowy organization in Taiwan. After undergoing a heart transplant, Xiang-Ying begins to experience vivid memories and emotions from her donor, a woman named Kaori Makimura. These memories lead her to Tokyo, where she seeks to uncover the truth about Kaori’s life and death.

In Tokyo, Xiang-Ying crosses paths with Ryo Saeba, a private detective known for his sharp skills and womanizing tendencies. Ryo, who was deeply connected to Kaori, becomes entangled in Xiang-Ying’s quest as she struggles to reconcile her identity as an assassin with the compassionate emotions inherited from Kaori’s heart. The series explores themes of identity, redemption, and the impact of past lives on the present, as Xiang-Ying navigates her dual existence and forms bonds with Ryo and his associates.

The supporting cast includes familiar characters from "City Hunter," such as Umibozu, a powerful martial artist, and Miki, a resourceful informant. These characters provide continuity for fans of the original series while contributing to the new narrative. The plot intertwines episodic detective cases with a larger, overarching story about Xiang-Ying’s journey of self-discovery and her attempts to break free from her violent past. The series balances action, drama, and emotional depth, offering a mix of intense confrontations and poignant moments as Xiang-Ying grapples with her conflicting identities and the legacy of Kaori’s heart.