Sophie Malaterre, an unemployed Canadian graphic designer, spontaneously accepts an offer to change her life: she exchanges her German apartment for one in Paris through an online portal called "Switch". She doesn't know her exchange partner, Bénédicte Serteaux, as the apartment keys are exchanged by mail.
On the day of arrival Sophie enjoys a stroll, but her nightmare begins when she wakes up the next day. A police task force storms into her new apartment, and she is arrested for murder and taken to the police station. A decapitated body has been found in a locked room: the body of Thomas, the life companion of her exchange partner Serteaux.
Sophie tries to convince the investigators that she is not Serteaux, but any evidence that would prove her true identity seems to have been destroyed overnight. In the apartment, investigators find a pass that identifies Sophie as Bénédicte Serteaux. Sophie is about to be committed to a psychiatric ward, but manages to escape. She calls her mother in Montreal and asks her to find anything that will prove her identity. Marianne Malaterre spares no danger to enter the house, paying a few minutes later with her life. She is killed by the paranoid Serteaux shortly after she sees the severed head of a man in the bedroom of the house.
Around the same time in Paris: Even Serteaux's mother Alice certifies to investigators that her daughter is unrecognizable on the mug shot, as had Serteaux's neighbors previously, leaving Sophie still misidentified.
The DNA analysis of the headless corpse is surprising: Thomas and Sophie are half-siblings. Thomas' mother explains that his father was an anonymous sperm donor. Sophie's father is obviously the donor (the Malaterre family lived in Paris until Sophie was twelve years old). And not only Thomas was created by artificial insemination; Sertaux's mother used the same Parisian laboratory.
Bénédicte Serteaux, a child of wealthy parents, had everything perfectly organized. She underwent facial surgery to resemble Sophie, and hired in Paris and Montreal two young people to act as decoys. She just had not expected that her opponent is against their arrest to defend.
Serteaux grew up without a father. Her mother hated her, she admits in a bitter showdown with her half-sister Sophie, as she beats her and breaks both her legs. Fortunately, Sophie, although in excruciating pain, can use her cellphone to call Commissioner Forgeat, who had come to believe in Sophie's innocence in recent days. The call location leads him and his colleagues to a factory, where they save Sophie just before she suffers a violent death.