In the shanty town of Tondo, Insiang Hilda Koronel works as a laundrywoman, while her mother Tonya Mona Lisa sells fish at a palengke. Tonya, who has weathered a painful rejection after her husband walked out with another woman, directs that hurt toward Insiang, treating her harshly and blocking any chance At finding love. Tonya is involved with Dado [Ruel Vernal], a butcher several years her junior, and she even ejects her sister-in-law and her family from their home, only to have Dado move in the next day.
Insiang’s car-mechanic boyfriend Bebot Rez Cortez sneaks into her house one night, pressing for sex to make up for a missed date. She refuses, warning him to leave before Dado—who slips out of his bedroom—wakes. Tonya learns of the affair and slaps Insiang repeatedly. Dado later meets Bebot and warns him to stay away from Insiang, insisting he has leverage over the girl and her mother. After Bebot relays Dado’s threat, Insiang confronts Dado about his meddling. He defends himself, claiming Bebot is with other women and that his warning was for her security, but Insiang does not buy it. That night, he chokes her into unconsciousness and carries her away.
Tonya wakes to find her daughter crying in pain and learns that she has been raped by Dado. When Dado returns home, Tonya throws objects at him and demands he leave. He admits to the sexual encounter but convinces Tonya that Insiang had tried to seduce him by bathing naked in his presence. Tonya then blames Insiang for the assault, comparing her to the girl’s father. Bebot agrees to elope with Insiang to prove his love, and they check into a cheap Binondo hotel, where they consummate their relationship. Insiang awakens the next morning alone, unsure of Bebot’s whereabouts.
She returns home, and Tonya forgives her on the condition that she work at the market to prevent another dalliance with Dado. That night, Dado sneaks into Insiang’s room and apologizes for raping her, claiming his love for her made him lose control, and explaining that he had only pursued Tonya to get closer to Insiang. He says he has given up his other girlfriends since falling for Insiang. She invites him to have sex again the following night. The next day, Bebot appears distant, and that night Insiang asks Dado to avenge her. Dado and his gang beat Bebot at the dump the next day. Tonya notices Insiang growing closer to Dado and confronts her, but Insiang repeats Dado’s claim that he seduced her to reach Insiang and that the two plan to marry. Enraged, Tonya stabs Dado to death as Insiang watches without visible emotion.
Sometime later, Insiang visits Tonya in prison. At first disinterested, Tonya reveals she has little guilt about killing Dado, saying she did it to prevent a life together for them. Insiang responds that she was disgusted by the rape and wanted Tonya to kill Dado in anger and jealousy. Tonya suggests Insiang should be overjoyed by her revenge, but Insiang tearfully seeks her mother’s affection and is met with coldness. She leaves, and Tonya, consumed by guilt, watches her daughter walk away behind the prison bars.
