Around 2043, overpopulation has caused a worldwide crisis, resulting in a strict one-child policy enforced by the Child Allocation Bureau. All but the eldest children are put into cryosleep. Electronic bracelets track all citizens.
Karen Settman dies while giving birth to identical septuplet sisters. Their grandfather, Terrence, names them after the days of the week and trains them to pose as a single individual named after their mother, leaving the house only on the day of their name. Terrence ensures they share information daily and replicate any physical accidents that alter their appearance. The sisters use wigs and makeup to cover any identifying features. After a young Thursday sneaks out of the apartment and injures herself skateboarding, Terrence amputates part of the index finger of each sister to mimic her injury.
In 2073, Monday prepares her disguise, nervous about giving a presentation. At a checkpoint, Monday runs into Adrian Knowles, a C.A.B. agent who flirts with her. At the bank, Monday’s co-worker, Jerry, a competitor for a promotion, hints at blackmailing her.
When Monday fails to return home, Tuesday retraces her steps. Tuesday learns that Monday got the promotion and met Jerry at a bar. Before she can investigate further, C.A.B. agents detain her and cut off her communications. In the C.A.B. facility, Adrian sees Tuesday escorted to a cell, where she meets Nicolette Cayman, head of the Bureau and a candidate for parliament. Cayman says she knows about Tuesday’s siblings, and, when Tuesday offers a bribe, Cayman reveals Monday offered the same deal. Cayman orders C.A.B. agents to assassinate Tuesday’s sisters.
C.A.B. agents use a severed eye to bypass a retinal scanner at the Settman home. The sisters kill the agents, but Sunday is killed. Learning the eye is Tuesday’s, the sisters suspect Jerry may have sold them out. The next day, Wednesday leaves without disguising herself and confronts Jerry. He reveals that “Karen” got the promotion when she sent millions of euros to Cayman to fund her campaign. After a C.A.B. sniper kills Jerry, Wednesday flees his apartment.
The others remotely guide Wednesday to safety but are interrupted when Adrian shows up at the sisters’ apartment, concerned about “Karen”. Surmising that Adrian has a relationship with one of them, Thursday convinces Saturday to leave with him. Saturday, who was a virgin, has sex with Adrian at his apartment. She covertly links their bracelets, as suggested by Friday, allowing Friday to hack into the C.A.B. On a video feed, the sisters believe they have found Monday in a holding cell. Meanwhile, C.A.B. agents corner and kill Wednesday. After Adrian leaves his apartment, C.A.B. agents arrive and kill Saturday after she tells her siblings Monday was dating Adrian. The sisters’ apartment is raided simultaneously by a C.A.B. squad led by Joe, the head of security of C.A.B. Admitting that she cannot survive on her own, Friday sacrifices herself by blowing up their apartment to allow Thursday escape and rescue Monday.
Adrian hears about the incident and rushes back to the Settman apartment. Thursday confronts him in his car, blaming him for her sisters’ deaths. Adrian now realizes “Karen” is an assumed identity for several siblings and claims to love Monday, whom he agrees to help rescue. Adrian sneaks Thursday into C.A.B. headquarters in a body bag. She secretly records a child undergoing cryosleep. Instead of being frozen, the child is incinerated. Adrian and Thursday discover Tuesday inside the cell with one of her eyes removed. They search for Monday, only to discover she has sold them out to Cayman.
At Cayman’s campaign fundraiser, Thursday and Monday fight in a women’s restroom, with Thursday accidentally shooting Monday. Meanwhile, Tuesday and Adrian broadcast Thursday’s video footage of the child’s incineration, leaving everyone at the event shocked and causing Cayman to faint in horror. The now traumatized crowd directs their attention on Cayman, who insists she did what was necessary (claiming that the extra siblings never suffered), and angrily confronts Thursday, choking her, before her bodyguards pull her off Thursday. Monday comes out of the restroom brandishing a gun. Joe shoots Monday, thinking she intends to kill Cayman, and Adrian kills Joe.
As the crowd flees, Monday reveals to Thursday that she is pregnant and asks her not let the C.A.B. take her unborn twins. Thursday realizes Monday sacrificed her sisters, replicating their grandfather cutting off their fingers, in order to protect her children. Monday dies from her wounds. The Child Allocation Act is abolished, and Cayman faces the death penalty. Thursday, Adrian, and Tuesday (with a new artificial eye) watch Monday’s and Adrian’s twins develop in an artificial womb. Tuesday declares Terry to be her new name, while Thursday declares hers to be Karen (henceforth becoming the “real” Karen Settman). As the camera dollies out, hundreds of babies are seen crying in one enormous ward, as nursing staff care for them.