Episode 4 of "Queen Mantis" opens with yet another flashback to the year 2001. I-shin's third victim is an abusive husband who menacingly warns his wife not to go to the police. In the present day, Na-hee and Sung-gyu find it perplexing that Su-yeol seems to possess an encyclopedic knowledge of every detail, from the hospital to the location of the well.
Meanwhile, Su-yeol questions Mr. Jeong, I-shin's father. Their relationship had always been strained, but Jeong had made it a point to check on his daughter, knowing full well the horrors she endured at the hands of her abusive husband. Jeong had even been present on the fateful day when I-shin took her husband's life. Su-yeol is incensed that Jeong didn't report his daughter's crime, but Jeong tearfully claims he couldn't bear to turn in his own flesh and blood.
Jeong assures Su-yeol that no one else is privy to the details of this first murder. He pleads with Su-yeol to abandon the case, but the young man is resolute, blaming himself for the chain of events that led to I-shin's crimes and those of her copycat.
Next, Su-yeol pays a visit to I-shin. She coolly offers to eliminate Gu-wan and repeatedly inquires about Jung-yeon. He sternly warns her against acting as if they share a close bond and steers the conversation back to the initial murder. Together, they deduce that the copycat must be someone intimately familiar with their domestic troubles.
Their first suspect is Park Min-jae, the son of their inebriated neighbor, Eon-bae. In a flashback, we witness I-shin arriving to kill Eon-bae, mistakenly believing he was abusing Min-jae. However, the bruises on Min-jae are revealed to be self-inflicted, a desperate cry for attention. I-shin comes to believe that Min-jae witnessed her actions and, out of a twisted sense of gratitude, is now emulating her.
Elsewhere, Jung-yeon and A-ra visit I-shin's grave to confirm the woman's demise. Unbeknownst to them, Min-jae watches from a distance and later comes to their rescue when a drunken Eon-bae launches an attack. Alone, Min-jae takes a pill and berates his father for the humiliation he has caused.
Next, he makes his way back to the women, his demeanor shifting to one of friendly familiarity upon discovering that Jung-yeon is the daughter-in-law of "Auntie I-shin." He meticulously clears his car of any incriminating items—pepper spray and a flashlight, eerily similar to those found on the copycat—and extends an offer to drive them home.
Su-yeol and Jeong-ho, who have been keeping a close eye on him, uncover his true profession: a private investigator. Tension spikes when Su-yeol spots Min-jae arriving with Jung-yeon and A-ra, but he forces himself to remain composed, inviting Min-jae inside so Jeong-ho can discreetly run his license plate.
Over drinks, Min-jae needles Su-yeol relentlessly, and it becomes unmistakably clear that he knows I-shin is the notorious Mantis. His admiration for the Mantis is palpable—he reveres her for eliminating abusers—and Su-yeol is left reeling when even Jung-yeon expresses a twisted form of sympathy.
Min-jae reminisces about how I-shin had been a maternal figure to him, shielding him from a hostile stranger and watching over him like a guardian.
Once Min-jae drops A-ra off at home, Su-yeol finally confronts Jung-yeon, his voice laced with anger over her snooping into I-shin’s identity. In a sharp, unexpected twist, she brings up his unresolved issues with his mother, insinuating that they’re somehow tied to his reluctance to have children.
Stunned, Su-yeol is left momentarily speechless, unable to pursue her as Jeong-ho interrupts with an urgent update: Min-jae’s address in Gwangju. Reviewing footage from his car’s black box, Su-yeol also catches Min-jae surreptitiously eavesdropping on his conversation with Jeong-ho just days prior.
Meanwhile, at the safehouse, I-shin is given a pill, but with a stealthy flick of her wrist, she spits it out unnoticed. Then, with calculated precision, she slips something else into the capsule, her eyes glinting with hidden intent.
As for Min-jae, he indulges in a bit of flirtation with A-ra before they go their separate ways. He's well aware that Jeong-ho is hot on his trail and cleverly manages to shake him off. Meanwhile, Su-yeol makes his way to Min-jae's Gwangju address. The place is a macabre miniature replica, showcasing all the Mantis murder scenes, along with a photograph of I-shin’s grave. Just as Su-yeol arrives, Min-jae shows up too, and a fierce fight ensues. Su-yeol gains the upper hand and accuses Min-jae of being the copycat killer. Min-jae merely smiles enigmatically and inquires if Su-yeol has met their mother.
Once again, Na-hee finds herself in the dark and pays a visit to Gu-wan. He pleads with her to loosen his restraints, claiming that "she" will end his life if she doesn't. Not to be outdone in the game of deception, I-shin pretends to be unconscious. When her guard, Woo-tae, attempts to administer CPR, she swiftly shoves a capsule filled with a toxic substance into his mouth. Woo-tae collapses, and I-shin seizes the opportunity to grab his wallet and make her escape.
Back to Min-jae, he's arrested but continues to goad Su-yeol as usual. However, as a seasoned private investigator, he has a habit of sharing his live location with his clients. He also has alibis for all the copycat murders. But Su-yeol isn't ready to let him off the hook just yet, suspecting that Min-jae might have planted his phone to create false alibis.
Unfortunately, the verification of Min-jae's alibis is put on hold when Gu-wan manages to overpower Na-hee and makes a desperate dash for freedom. While Na-hee and Su-yeol scour the area in search of him, someone watches from the shadows of a car. Seizing the moment, the car speeds up and runs Gu-wan over before driving off into the night. Su-yeol rushes to his side, but Gu-wan, in his last breaths, accuses Su-yeol of being just like I-shin and wanting him dead. Before Su-yeol can respond, Gu-wan takes his final breath.
Once Woo-tae regains consciousness, he alerts Jeong-ho, who uncovers I-shin's sinister secret—she had been raising poisonous oil beetles in the false ceiling, which is how she managed to incapacitate Woo-tae.
At the morgue, Su-yeol chastises Na-hee for not trusting him and for approaching Gu-wan alone. But Na-hee has no time for his reproaches; she's determined to pursue the hit-and-run driver, believing him to be the copycat killer. Su-yeol, however, is skeptical, thinking she's driven by a personal vendetta. Upset by his lack of understanding, Na-hee storms off in a huff.
Su-yeol's frustration mounts when he learns that I-shin has managed to escape. As Episode 4 of Queen Mantis draws to a close, I-shin makes a chilling appearance at Jung-yeon’s doorstep, a sinister smile playing on her lips.
