While caring for her niece, Sofia, the narrator notices a birthmark identical to one on her husband David’s back. Alarmed, she starts suspecting an affair between David and his sister, Fiona, Sofia’s mother. “I need to know if Fiona is his mistress,” she thinks, and decides to conduct a secret DNA test to uncover the truth.

Fiona and Sofia’s frequent visits heighten the narrator’s anxiety. Despite her suspicions, she hides her feelings, secretly collecting DNA samples from David and Sofia. When Fiona visits again, the narrator’s unease turns to anger. Eventually, unable to contain her emotions, she confronts them, accusing David of fathering Sofia.

David denies the accusation, insisting Fiona is his sister. Still doubtful, the narrator reveals she already conducted a DNA test. Fiona then shows the narrator the same birthmark, explaining it is a family trait. She also reveals Sofia’s father had passed away before her birth.

The DNA test results confirm that David is not Sofia’s father. The family clears the misunderstanding, and the narrator feels relieved as her family remains intact. Months later, after giving birth to her son Zack, she is warmly embraced by David’s extended family, bringing closure to the incident.