Around the year 2043 overpopulation has caused 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 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, and to leave the house only on the day of their name. To safeguard their secret, Terrence ensures they share information 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.

In 2073, 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, but not seriously. 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.

When Monday fails to return home, Tuesday retraces her steps. 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, 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. Thursday ambushes the agents as they arrest the other siblings. The sisters kill the agents, but Sunday dies. 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 "Karen" got the promotion when she 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 Adrian leaves his apartment, C.A.B. agents kill Saturday as she tells them Monday was dating Adrian. Reasoning that she cannot survive on her own, Friday sacrifices herself to give Thursday a chance to rescue Monday when C.A.B. agents again storm the sisters' apartment.

Adrian hears about the incident and rushes to the apartment. Thursday takes him hostage and blames him for her sisters' deaths. He is at first confused but claims to love Monday after realizing they are siblings. 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, she is incinerated, which Thursday records. After overpowering the guards, Adrian and Thursday proceed to rescue Monday. However when they reach her cell, they learn that it's actually Tuesday. They deduce that it is Monday who has sold them out to Cayman.

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 Thursday. As the crowd flees, dying Monday reveals to the others that she is pregnant. 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.

https://en.wikipedia.org/wiki/What_Happened_to_Monday
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