The story follows Mia, a seven-year-old girl who returns from her first sleepover visibly frightened and physically ill. She tells her father that Gregory Hale gave her a bitter drink and warned her not to tell anyone. At the hospital, doctors discover a plant-based substance affecting her heart and begin treatment. Police become involved, and Mia’s father realizes Gregory is someone from his past. Seventeen years earlier, he had helped Rachel, a college classmate and Gregory’s former girlfriend, report Gregory’s abuse, leading to his expulsion. Rachel later died from what was believed to be an accidental medication reaction, raising suspicions about Gregory’s involvement.

Gregory’s ex-wife, Laura, then reveals that he had researched poisonous plants and had threatened to take away what Mia’s father loved most. Investigators discover that Gregory deliberately entered the family’s life, enrolled his son Owen at Mia’s school, and secretly tracked the family for years. A notebook identifies people he believed had wronged him, including Rachel and Mia’s father, and evidence connects him to multiple suspicious deaths. Gregory is arrested, while Mia begins trauma therapy and struggles with fear and anxiety after the attack. A recorded conversation also reveals that Gregory manipulated his own young son into believing Mia’s father was responsible for destroying their family.