In 2073, overpopulation causes a worldwide crisis, resulting in a strict one-child policy enforced by the Child Allocation Bureau. When multiple children are born to one mother, all but the eldest are put into cryosleep. Karen Settman dies when she gives 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. To safeguard their secret, Terrence ensures they share information at a meeting on a daily basis and replicates any accident that affects one of the sisters' physical appearance. The sisters develop a system of wigs and makeup to cover any identifying features. Terrence reminds them of the importance of going out only one at a time, and that what happens to one of them must happen to all of them. When Thursday breaks the rules by going out on a day that isn't hers, she injures one of her fingers, severing it at the first knuckle. A disappointed Terrence is forced to remove the same portion of finger from all the girls to maintain a single appearance, starting with Monday.
Decades later, the sisters have all managed to survive without suspicion. As Sunday returns from her job at a bank, she sees C.A.B. agents detain a child as onlookers protest. The siblings watch the recorded incident and debate turning themselves in, particularly Thursday, who wonders if being put into cryosleep would be so bad; she wouldn't mind being woken up even a hundred years in the future if it means being able to be her own person in public and find a romantic partner. Others, like Saturday, are upset at the thought of being separated, and the responsible eldest sister, Monday, insists on the importance of working together as a unit. After Monday's admonishment, everyone agrees to continue their act. The following day, Monday prepares her disguise as Karen, 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.
The day wears on, but Monday doesn't return home. This worries the rest of the sisters, who know that Monday is the most responsible and has never missed a meeting. Tuesday retraces her steps the next day. Tuesday learns 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. Adrian sees Tuesday being escorted to a cell, where she meets Nicolette Cayman, head of the bureau and a candidate for parliament. Cayman explains she knows about Tuesday's siblings, and, when Tuesday offers a bribe, reveals Monday offered the same deal. Cayman orders C.A.B. agents to assassinate Tuesday's sisters as Tuesday is presumably killed.
C.A.B. agents use Tuesday's severed eye to bypass a retinal scanner. Thursday ambushes the agents as they arrest the other siblings. The sisters kill the agents, but Sunday dies after being hit by gunfire. 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 the sisters got the promotion when "Karen" sent millions of euros to Cayman to fund her campaign. After a C.A.B. sniper kills him, Wednesday kills several C.A.B. agents and flees.
As the others remotely guide Wednesday to safety, Adrian shows up at the apartment, concerned about "Karen". Thursday convinces Saturday to leave with Adrian, who has had a long-term sexual relationship with one of the sisters. Pretending to be Karen, Saturday has sex with Adrian and covertly links their bracelets, allowing Friday to hack into C.A.B. headquarters. On a video feed, they apparently find Monday in a holding cell. Meanwhile, C.A.B. agents corner and kill Wednesday after the latter makes a desperate leap across the rooftops. After Adrian leaves his apartment, C.A.B. agents kill Saturday after she reveals that Monday was the one dating Adrian. Reasoning that she can not survive on her own, When C.A.B. agents storm the apartment, Friday sacrifices herself to give Thursday a chance to escape and save Monday, and she blows up the apartment.
Adrian hears about the incident and, distraught, rushes to the apartment. Thursday takes him hostage and accuses him of selling Monday out, blaming him for the deaths of her sisters. He is at first confused but claims to love Monday after realizing they are siblings. Realizing that he is innocent, Thursday convinces Adrian to help by telling him Monday is still alive. Adrian sneaks Thursday into C.A.B. headquarters in a body bag. As she is prepared for disposal, a child undergoes cryosleep. Instead of being frozen, it is incinerated, which Thursday records. After overpowering the guards, Adrian and Thursday discover that Tuesday is alive, only missing her eye. When Thursday asks Tuesday what happened to Monday, it is revealed that Monday indeed bribed Cayman with a massive campaign donation — to get rid of her other sisters and take on the identity for herself.
Thursday confronts Monday in the bathroom. Monday retorts that she was always the best Karen Settman, and deserved to have the life for her own; her sisters were just "afterbirth." Monday resented the other sisters for not being as responsible as her, especially Thursday, whose carelessness cost everyone part of a finger. After a scuffle, Thursday shoots Monday and leaves her for dead. As Cayman hosts a fundraiser, Tuesday and Adrian broadcast Thursday's video footage. The crowd turns on Cayman, who insists she only did what was necessary. Monday staggers into the fundraiser, but a C.A.B. agent shoots her before she can kill Cayman. As the crowd flees, Monday reveals to the others that she was pregnant and pleads with Thursday to take care of her children as she dies. In the aftermath, the Child Allocation Act is repealed and Cayman faces the death penalty. Thursday, Adrian, and Tuesday watch Monday's and Adrian's twins develop in an artificial womb. Tuesday and Thursday rename themselves Terry and Karen, respectively.
