Some years before the film, a controversial test costing one fingernail determines whether couples are truly in love. The test has produced largely negative results, fueling a rise in breakups and divorces.
Anna is an unemployed schoolteacher who lands a job as an instructor at the Love Institute, a one-year-old company founded by Duncan, where researchers and instructors guide couples through relationship-strengthening exercises to help them pass the test. Duncan believes that continued research will yield more positive results. Anna initially keeps the hire a secret from her boyfriend Ryan.
Anna shadows Amir, and the two form a rapport as they guide clients through the exercises. Although Amir claims he and his girlfriend Natasha have also passed the test, his fondness for Anna is clear. Meanwhile, Anna secretly runs experiments with an unsuspecting Ryan to test their love. She eventually confesses the job to Ryan, explaining that she feared taking the job would strain their relationship, even though it clearly did.
After Anna first administers a test to a promising client couple, the result is negative, and she is shaken. Amir tells her she’ll get used to it. At a workplace party, Amir’s girlfriend Natasha is puzzled when Anna mentions his gluten intolerance, prompting Anna to question whether she and Amir feel the same way about each other.
On another day, Amir tears out his own fingernail to calm a nervous client. The client couple receives a positive result, delighting everyone. Seizing the opportunity, Anna secretly tests Amir’s fingernail against her own. The result is 50%, meaning one person is more in love with the other. When she asks Duncan if a person can have a positive result with two different people, he rejects the hypothesis. She later convinces a hesitant Ryan to take the test with her again, and they pass.
The next day, Anna confesses her findings to Amir, who confirms his feelings for her when she suggests that he is in love with her, but she does not love him in return. Later that night, Anna and Amir kiss and sleep together. She voices hesitancy about continuing her relationship with Ryan, stating, "Sometimes, being in love is lonelier than being alone."
Amir reveals that he began working at the Love Institute because he felt something was wrong after never seeing any positive results. When Anna decides she wants to stay with him, Amir says they shouldn’t be together and they must obey the test. She stays over for the night, but awakens to find herself removing more of her fingernails in his kitchen out of heartbreak. Amir stops her and offers comfort. As they wait for her fingers to heal, they share a quiet, hopeful smile.
