The Mantis: Original Sin, Season 1 (2025) begins with Detective Cha Soo-yeol investigating a brutal murder case that eerily mimics the modus operandi of his estranged mother, Jung Yi-shin, known as the "Mantis," a notorious female serial killer who murdered five men decades ago. The police, desperate to catch the copycat killer, decide to consult Jung Yi-shin, who is still imprisoned. Despite his deep hatred and resentment toward his mother, Cha Soo-yeol is assigned to work alongside her to solve the case.

The first sequence shows Cha Soo-yeol arriving at the prison to meet Jung Yi-shin. Their reunion is tense and emotionally charged, with Cha confronting his mother about her past crimes and the pain she caused him. Jung remains cold and calculating but agrees to help, seeing this as an opportunity to manipulate the investigation for her own ends. They begin reviewing the details of the new murders, noting the similarities to Jung’s original killings.

As the investigation progresses, Jung provides insights into the psychological profile of the copycat killer, revealing her intimate knowledge of the original crimes. Cha Soo-yeol struggles to balance his professional duty with his personal feelings, often clashing with his mother over the direction of the case. Meanwhile, flashbacks reveal Jung’s past murders, showing her methodical and ruthless approach to killing the five men, each murder carefully planned and executed with chilling precision.

The next major event involves the discovery of a new victim whose death matches the Mantis’s signature style. Cha and Jung visit the crime scene together, where Jung points out subtle clues that the police had missed. This leads to a breakthrough in the investigation, as they identify a pattern in the victims’ backgrounds that suggests the copycat is targeting men with specific traits. Cha begins to suspect that the copycat killer may have a personal vendetta linked to Jung’s past.

Tensions rise when Cha Soo-yeol’s colleagues express distrust toward Jung’s involvement, fearing she might be manipulating the investigation or even orchestrating the new murders from prison. Cha defends his mother, insisting on her cooperation, but privately he remains wary. Jung uses her influence to gain access to restricted case files, uncovering information that points to a conspiracy involving powerful figures connected to the original murders.

A critical turning point occurs when Cha Soo-yeol narrowly escapes an ambush set up by the copycat killer. The sequence is detailed: Cha is investigating a lead alone at night when he is attacked by an unknown assailant. He fights back, using his training to disarm and incapacitate the attacker, who escapes but leaves behind evidence linking them to the copycat murders. This close call intensifies the urgency of the investigation and deepens Cha’s resolve to catch the killer.

Jung Yi-shin reveals to Cha that the original five men she killed were involved in a dark secret that destroyed her family, explaining her motives for the murders. She confesses that the copycat killer is likely someone seeking revenge or trying to expose the same secret. This revelation shifts the investigation’s focus toward uncovering the hidden connections between the victims and the powerful individuals behind the scenes.

In the penultimate episodes, Cha and Jung work together to set a trap for the copycat killer. They use Jung as bait, anticipating that the killer will attempt to eliminate her to stop the investigation. The sequence unfolds with careful planning: surveillance is set up around Jung’s prison visits, and undercover officers are deployed. When the copycat makes a move, a tense confrontation ensues. The killer attacks Jung during a prison transfer, but Cha and the police intervene just in time, leading to a violent struggle that ends with the copycat’s capture.

The final episode reveals the identity of the copycat killer: a man connected to the original victims and the conspiracy Jung exposed. He confesses that his motive was to avenge the injustices done to his family by the same men Jung killed. Cha Soo-yeol confronts him, grappling with the moral complexities of justice and revenge. Jung, meanwhile, faces the consequences of her past actions but gains a measure of closure through the resolution of the case.

The season closes with Cha Soo-yeol visiting Jung in prison one last time. Their relationship remains strained but shows signs of tentative understanding. Cha acknowledges the pain of his past but commits to moving forward. Jung, in turn, accepts her fate but hints at deeper secrets yet to be uncovered, setting the stage for potential future developments.

Jung I Sin, also known as the Mantis, is a female serial killer who brutally murdered five men two decades ago. She is the mother of Cha Soo-yeol, a police officer who has loathed her his entire life. When a copycat killer emerges, Soo-yeol enlists her help to catch the culprit, setting up a tense and complex mother-son dynamic central to the story.

Cha Soo-yeol is the son of Jung I Sin, the Mantis. He has hated his mother for years due to her past as a serial killer. Despite this, he must work with her when a new series of murders mimicking her style occur, creating a fraught and emotionally charged partnership.

Yoon Hee-jin is a brilliant but troubled forensic pathologist and the mother of Park Ji-won. Set 20 years before the original series, she investigates a series of deaths she believes are connected, which strains her relationship with her daughter. Her obsession with uncovering the truth leads her into dangerous territory that ultimately shapes the events of the original story.

Detective Choi Min-ho is a reluctant ally to Yoon Hee-jin. He harbors his own secrets and their relationship forms the emotional core of the series, portraying two broken individuals who find understanding in each other despite knowing they cannot be fully saved.

The hospital director, portrayed by Yoo Jae-myung, presents a benevolent façade but hides a far more sinister nature. His quiet menace adds tension to the series and plays a key role in the unfolding conspiracy that Yoon Hee-jin investigates.

The TV show The Mantis: Original Sin (2025) is not family friendly and is generally unsuitable for children or sensitive viewers due to its intense crime thriller content involving serial killings.

Potentially objectionable or upsetting aspects include:

• Graphic violence and murder scenes, as the story revolves around a serial killer and copycat killings.
• Psychological thriller elements with dark, intense themes that explore trauma, obsession, and complex family dynamics.
• Disturbing subject matter related to crime, death investigations, and the emotional toll on characters.
• Strong emotional tension and suspense that may be unsettling for sensitive viewers.

The series is a mystery-thriller with a mature tone, focusing on a cop working with his mother, a notorious female serial killer, which inherently involves mature and potentially distressing content.