An unnamed widow sustains her life by selling medicinal herbs in a quaint town in southern South Korea while also offering unlicensed acupuncture treatments to women in the community to help them forget painful memories. Raising her only son, Yoon Do-joon, who is shy yet prone to violence if ridiculed for his intellectual disability, she is fiercely protective of him. Although she often scolds him for socializing with a local thug named Jin-tae, their lives take a dark turn when Do-joon vandalizes a car after a near-accident. As a result, he faces a lawsuit, expanding his mother’s burdens.
The tragedy escalates when Do-joon, returning home from a night out, follows a high school girl named Moon Ah-jung to an abandoned building. The next day, her lifeless body is discovered on the rooftop, sending shockwaves through the community and igniting a media frenzy that pressures the police to act swiftly in uncovering her killer. Circumstantial evidence leads to Do-joon’s arrest, though his mother steadfastly believes in his innocence. She embarks on a quest to exonerate him, but faces significant challenges from her self-centered lawyer and a community that quickly turns against her son.
In a desperate attempt to find the truth, the widow breaks into Jin-tae's home, convinced he is the true culprit. Though she retrieves what she thinks is a blood-stained golf club, it is revealed to be only lipstick, leading to Jin-tae offering his services for a fee instead of facing charges. As the mother investigates further into Ah-jung’s life, she learns that the girl had a reputation for being promiscuous and had been involved with Jong-pal, a boy who had escaped a sanatorium. In a startling visit to Do-joon in prison, he recalls a traumatic incident from his childhood, where his mother attempted to end their suffering by poisoning them, stating, > "I never want to see you again."
Through her persistent digging, the mother discovers that Ah-jung had frequent nosebleeds and owned a cellphone containing potentially incriminating pictures. After witnessing two men attacking Ah-jung’s friend looking for the phone, she hires Jin-tae to track them down. She ultimately uncovers that the phone was hidden at Ah-jung’s grandmother's house.
During a poignant revelation, Do-joon identifies an elderly man in photos from Ah-jung's phone, a junk collector he remembers. Pretending to offer medical care as a way to glean information, the collector divulges a disturbing truth: he witnessed Do-joon accidentally kill Ah-jung after she taunted him with names. Unable to bear the weight of this reality, the mother suffers a breakdown, resorting to violence against the collector to protect her son.
As events unfold, the police claim to have captured the true murderer, Jong-pal, who insists the blood on his shirt is from a consensual encounter with Ah-jung. Overcome with guilt, the mother visits him, understanding the anguish of a son with no advocate to champion him.
With Do-joon’s release from prison impending, Jin-tae retrieves him, but the pair stops to sift through the ashes of the burnt-down collector's home. During a moment of introspection at dinner, Do-joon reflects that perhaps Jong-pal meant to help Ah-jung. Just before boarding a bus for a "Thank-You Parents" tour, Do-joon returns his mother’s acupuncture kit, found amid the wreckage. This prompts a shocking realization for her as she grapples with the past. Ultimately, she chooses to erase the memories of their shared guilt, participating in the celebratory dance with other parents on the bus.
