"What Happened to Monday" is a dystopian science fiction film set in a future where overpopulation has led to strict government policies limiting families to one child. The story follows seven identical sisters who are named after the days of the week. They live in secrecy, sharing the identity of their one legal sibling, Monday, to avoid detection. When Monday goes missing, the remaining sisters must navigate a dangerous world to uncover the truth behind her disappearance. As they face challenges from the oppressive government and uncover dark secrets, they fight to protect their family and their right to exist.