While walking home from work, Thana, a mute seamstress in New York City’s Garment District, is raped at gunpoint in an alley by a mysterious, masked attacker. She survives and makes her way back to her apartment, where she encounters a burglar and is raped a second time. Thana—her name an allusion to Greek god of death Thanatos—hits this second assailant with a small red glass apple, then bludgeons him to death with an iron, and carries his body to the bathtub. She goes to work the next day, and after encountering working with an iron, watching her boss Albert rip a shirt off a mannequin, she goes into shock state, which worries her co-workers. However, when she looks at the trash bin at her office, she decides to dismember the burglar’s corpse and throw the parts away in various locations of the city.
After being sent home, she dismembers the burglar’s body, then keeps his .45 caliber pistol, puts the pieces into plastic garbage bags, and then stores them in her fridge. After cleaning her bathtub, she decides to take a shower, but as she strips, she begins to hallucinate the first attacker in the mirror grabbing her breast. This puts her into shock, and she notices that organs and body fluids from the burglar are overflowing in the drain. Her nosy neighbor, an old, recently widowed woman named Mrs. Nasone who is the landlady and owns a small dog named Phil, also starts to notice her odd behavior.
On her walk home from work the next day, Thana is noticed by a leering young man on the street while she is disposing of one of the bagged body parts; thinking that she accidentally dropped the bag, he retrieves it, frightening her. He chases her through the alleys of the city and, fearing another sexual assault, she fatally shoots him when she is cornered by him. The event furthers her impulse for vengeance. While running home from the incident, the landlady Mrs. Nasone notices she ran up the stairs violently and started throwing up. She insists in calling a doctor for Thana, and Mrs. Nasone’s dog, Phil, starts to become attracted to her fridge. Thana escorts her out of her apartment while still in shock.
As the limbs start to bring the attention of the media, Albert brings her into his office and notices she hasn’t been sewing enough and hasn’t been feeling well lately. He decides to invite her to a Halloween party that he is throwing for work, and tells her that there will be “many boys there [her] age” while stroking her neck. She responds to him in writing, saying “I’ll try”. As Thana’s vengeance increases, she starts regularly targeting and killing several men, such as a fashion photographer, a pimp who assaults a prostitute because of debt, several members of a gang, a Saudi Arabian businessman and his limousine driver, and even drives a recently dumped salesman to suicide after her gun jams.
Her boss, Albert, notices her ditching work after going to dinner with her co-workers, resulting in her co-workers having to finish her work. However, she promises to go to the party with him in exchange for staying out of trouble for her absence. Thana notices that Mrs. Nasone’s dog, Phil, is attracted to the smell of the burglar’s dismembered limbs. Thana takes Phil for a walk then ties him to a post in the park. She leaves a note saying that Phil ran away but will probably find his way back home soon.
With Albert, Thana attends the costume party dressed as a nun with red lipstick. Meanwhile, Mrs. Nasone enters Thana’s apartment and finds the burglar’s dismembered head. She jumps to the conclusion that Thana killed her dog, and tells police that she is at a party with her co-workers. At the party, Albert takes Thana upstairs in private and tries to seduce her. In revenge for his borderline-sexual behavior towards her in the past, she shoots him. The party stops and her co-workers run upstairs to Thana, but quickly realize that she is the murderer when she steps out of the room with her pistol. Thana then begins a shooting spree, targeting many of the men present. Her co-worker Laurie picks up the knife from the cake table and stabs Thana in the back. Thana turns around and points the gun at Laurie, but hesitates; she screams in pain, “Sister!” then falls to the ground and dies.
After the party massacre, Mrs. Nasone is seen crying in memorial for her husband and her dog Phil. But outside her apartment door, Phil has returned and is shown running up the stairs and scratching at the door.