In the near future, Sarah is a depressed alcoholic in a lackluster relationship with her boyfriend Peter and generally disconnected from her pestering mother. One day, Sarah awakens to find a pool of blood in her bed and later learns she is terminally ill with a 2% chance of surviving. To save those she cares about from the pain of losing her, Sarah opts for a cloning procedure of herself to take her place. She soon dubs the clone "Sarah's Double" and gives her the basic knowledge of her interests and lifestyle. Months later, however, Sarah learns she is miraculously in remission from the disease and is going to live. When she goes to her mother's house to share the good news, she finds Peter and Sarah's Double there and is furious to discover that Sarah's Double had been in contact with her mother for quite some time against her wishes. The truth now exposed, Sarah tries to return things to normal and demands her clone to be "decommissioned", only to be rejected by Peter who, as well as her mother, both prefer the clone.
Sarah is told that Sarah's Double requested to stay alive, meaning that, by law, they would have to participate in a publicized duel to the death they must prepare for in a year. Having to pay for her clone as well as herself, Sarah soon takes up self-defense and combat classes with the trainer, Trent. For the next year, Sarah has improved herself both physically and mentally, learning to tolerate violence and how clone duels operate. To show her thanks, she teaches Trent hip-hop dancing. Feeling confident, Sarah meets up with Peter to state that she means no ill will but promises to unapologetically kill her clone.
While in the middle of a test with Trent, Sarah spots her clone watching from outside and chases her to a nearby playground. There, they talk over their situation before Sarah's Double takes Sarah to a support group for people who survived their duels. Outside, the two bond and agree to escape past the border to live their lives. The next morning, the two hike through a forest where Sarah discovers that Sarah's Double has poisoned her water. Sarah's Double eventually shows up late to the duel alone, lying that she is the original and stating that the "clone" has fled. After a thorough investigation, she is free to completely take over as the only Sarah. However, Sarah's Double is soon left feeling just as depressed and unfulfilled as the original Sarah, carrying her burdens such as the unfiltered Peter and her doting mother (who both know she is the clone). While out driving, Sarah's Double stops in the middle of a traffic circle and cries.