A young Taiwanese assassin codenamed "Glass Heart" attempted suicide by jumping from a building, resulting in her heart being pierced by a metal fence. Miraculously, she survived thanks to a heart transplant. During her recovery, she began having strange dreams that led her to Japan in search of the heart donor, who turns out to be Kaori Makimura, the former partner of City Hunter Ryo Saeba. The author noted in the first tankoubon volume that Angel Heart shares the same characters as City Hunter but is not its continuation. The events take place in a parallel universe, meaning they are unrelated to City Hunter.