ClaireBell

Bell is wrongfully convicted of drug possession during her graduation party at a club, leaving her sick father behind. Her cellmate Kae introduces her to the prison's hierarchy, including the influential 3D gang, the prison economic system, and the infamous 'Rabid Dog' inmate whom everyone stays away from. Bell begins working at the library, and meets 'Rabid Dog' by chance. She later learns that the inmate's name is '19 Stab' Claire, a young woman convicted of brutally murdering her physics teacher years ago. A new prison officer misuses her power, and is almost strangled to death by Claire.

Bell becomes a target of harassment by DeeDee, one of the 3D gang members, due to a small mistake. While being chased by the gang at the canteen, Bell is saved by Claire, who then invites her to share a meal together. Later, DeeDee and her sister Duen, with the help of the new prison officer, enters Bell's cell to take revenge, while the prison officer enters Claire's cell to 'teach her a lesson'.

Claire subdues the new prison officer and saves Belle in time. Wichai, the prison warden, reluctantly brings in his son, Porche, as the new prison officer.

Dao's teenage son visits her for the first time. Bell attempts to apologise to DeeDee, but unintentionally makes things worse for herself. DeeDee and her gang attempts to make a jump on Bell, who hides in the prison's abandoned toilet. There, she meets Claire and they share a conversation. Later, Mint visits Claire to push for an explanation on why the latter murdered her brother.

Elsewhere, Bell's boyfriend Top, who convinced her to reduce her sentence by pleading guilty, begs his father to get Bell out of prison. His father berates him and refuses his request. To Bell's devastation, Top lies about his involvement with Bell on national TV. When DeeDee and Duen attempt to take revenge on Bell again, they accidentally injure Pa'Pon, Claire's elderly cellmate.

DeeDee and Duen narrowly avoid Claire's wrath. Pa'Pon received minor injuries, and Porch discovers that the elderly woman has Alzheimer's. Claire threatens DeeDee in the bathroom for harming Pa'Pon. Bell finds out that her father's nurse has ceased her services under Top's instructions. DeeDee's hazing on Bell continues. Pa'Pon's Alzheimer's symptoms worsen, but shares a tender moment with Claire after she suddenly remembers the reason behind her manslaughter charge. Kae proposes a money lending scheme to DeeDee and Duen in return for them to stop harassing Bell. In the abandoned toilet, Claire and Bell have a heartfelt conversation and give in to their feelings for each other.

Later, Top visits Bell and informs her that her father has passed away. Claire discovers Pa'Pon's lifeless body in their cell.

Pa'Pon dies from an accidental overdose, and Claire blames Wichai's negligence for the tragedy. Bell pays a bribe to attend her father's funeral thanks to Kae's loan. When Bell witnesses DeeDee's gang beating up an inmate for failing to pay back their debt, she confronts Kae for being complicit in the violence. An upset Kae confesses her feelings to Bell, and is immediately rejected.

Released from solitary confinement, Claire mourns Pa'Pon, and meets Bell at the abandoned toilet. They comfort each other in their respective grief, and Claire reveals that she took the fall for her younger sister Natty, who committed the actual murder. They make love for the first time. Unbeknownst to them, Kae witnesses the entire scene. Mint bumps into Natty while attempting to visit Claire.

Claire and Bell bring their relationship into the open. Dao confronts DeeDee and Duen about running their money lending business behind her back, and Kae gains more power with Dao out of the picture. Porsche reports his father's negligence on Pa'Pon's case to the prison director. Claire breaks Bell out of her cell and teaches her how to short circuit the prison's electrical system. They roam about the grounds before Claire proposes to Bell with a vine ring, and they have an imaginary wedding in the greenhouse.

Elsewhere, Natty breaks down in front of her parents from the weight of what happened to Claire. Later, rumours circulate that Natty is the real '19 stab' killer. It is revealed that Kae overheard Claire's confession and had instructed her friend to spread the rumour. Mint provokes Claire with the new information during her visit.

In a flashback, Claire drops out of school to sell secondhand goods. Natty convinces her physics teacher Maed to tutor her so she could join the Science Olympiad. During a study session, Maed sexually assaults Natty, who stabs him with a pair of scissors in defence. Claire rushes to the computer lab when she receives Natty's call, and finds her sister covered in blood next to Maed's lifeless body. She makes Natty promise to keep the incident a secret, and sends her home before smearing her face and body with Maed's blood.

In the present, Claire accuses Bell of revealing her secret. She lands herself in solitary confinement for a month after punching Wichai for rejecting her job request. Bell borrows money from Kae to grow roses in the greenhouse. As Claire begins to lose her mind, Bell sends her a tray of bread and a sketchbook reminiscing their time together. Later, Bell is attacked by DeeDee's gang under Kae's instructions for not paying back her loan in time. Kae cements her power by pushing DeeDee and Duen out of the business. Claire blackmails Wichai and has Bell transferred to her cell.

Mint accidentally obtains Maed's phone after a failed attempt to gain information from Natty.

Top pressures Beam to confess to putting the drugs in Bell's handbag at the club. DeeDee and Duen reconciles with Dao. Claire and Bell take up jobs under Dao's supervision to pay off Bell's debt. Dao finds out she's pregnant and confronts Kae for swapping out her birth control pills. Wichai pressures her to get an abortion. Beam visits Bell to tell her that he'll confess, and reveals that the drugs belonged to Top. After getting thrown into solitary confinement for attempting to shiv Kae, DeeDee and Porsche grow closer. Claire and Bell spend a night outdoors to celebrate her impending freedom. Natty reveals the truth to her parents.

Dao decides to keep the baby. Kae's friend stops paying her her cut of the scheme that landed her in prison. Unable to pay their cut, her underlings leave her to collect her debts alone.

Mint discovers lewd pictures of his students on her brother's phone, along with the video of his sexual assault on Natty. She meets Natty to pass her back the phone, who hands it in to authorities as evidence of self-defense, thus clearing Claire of all charges. On the day of Bell's release, Kae tampers with the canteen food, triggering a severe allergic reaction from Bell. Claire brutally attacks Kae, and is sentenced to three years in prison for attempted murder. Porsche resigns from his position, and Wichai is subsequently fired for inappropriate relations with an inmate.

Bell visits Claire in prison, and promises to wait for her, but is rejected by Claire. Bell confronts Top at his home about the drugs. While he's intoxicated in the garden, Bell short circuits the electrical system and burns down the house, prompting a search from the police during the fire rescue, who finds large amounts of drugs in Top's study room.

Three years later, Duen is in charge of the kitchen, DeeDee is more in control of her temperaments, and Kae shares a cell with an aggressive inmate. Dao's infant son visits her as Claire is leaving. Claire's family greets her at the prison entrance. As they are leaving, Bell calls out to Claire from a distance, and they happily embrace.