In the bustling streets of Shinjuku, Tokyo, a young female assassin known only as Glass Heart carries out her final mission. Trained from childhood by a shadowy underworld organization, she has been conditioned to kill without emotion, but the weight of her actions becomes unbearable. She attempts suicide by leaping from a building, only to survive after receiving an emergency heart transplant. The donor is Kaori Makimura, a bright and warm hearted young woman who was recently killed in a traffic accident. Unbeknownst to the medical staff, Kaori was the longtime partner of Ryo Saeba, a legendary private detective and “sweeper” who takes on dangerous cases across the city while operating from a dilapidated office with a “XYZ” answering machine for clients in need.

After recovering, Glass Heart abandons her past and tracks down Ryo, drawn by unfamiliar emotions and fragmented memories that seem to belong to Kaori. She takes the name Xiang Ying, and despite Ryo’s initial resistance, she becomes his new partner. Together, they work as unlicensed fixers, handling everything from extortion and kidnapping to murder investigations, often clashing with organized crime and corrupt officials. Throughout the series, Xiang Ying struggles to reconcile her lethal training with Kaori’s gentle heart, which increasingly influences her decisions. She experiences tastes, fears, and attachments that are not her own, including a deep protective instinct toward Ryo and a lingering sadness over Kaori’s unfinished life.

Ryo, meanwhile, continues to mask his grief with cocky charm, chain smoking, and a habit of flirting with every woman he meets, but he quietly accepts Xiang Ying as a second chance to honor Kaori’s memory. Their partnership is tested by recurring threats from the syndicate that created Glass Heart, which wants her back or eliminated. A major narrative arc follows the hunt for her former handler, a ruthless operative known as the Professor, who implanted her with a failsafe poison capsule that still lies dormant in her body. Another extended arc involves a mysterious young girl who claims to be Kaori’s illegitimate daughter, forcing Ryo and Xiang Ying to investigate Kaori’s hidden past and protect the child from traffickers.

Supporting characters include the bumbling but well meaning police detective Hideaki Todo, who often stumbles onto Ryo’s cases; Miki, a streetwise informant and owner of a cat themed bar; and a retired assassin who becomes a reluctant ally. As the series progresses, Xiang Ying gradually learns to embrace both her own identity and Kaori’s lingering presence, realizing that her second chance at life means protecting those Kaori loved. The season culminates in a final confrontation with the Professor, where Xiang Ying must choose between her programmed instincts and the humanity growing inside her. Ryo, standing beside her, finally acknowledges that Kaori’s heart found its way home.