A 17-year-old Manhattan student, Lisa Cohen, shopping on the Upper West Side, interacts with bus driver Gerald Maretti as she runs alongside his moving bus; he allows himself to become distracted, leading to a fatal accident by missing a red light, in which a pedestrian, Monica Patterson, is hit by the bus and subsequently dies in Lisa's arms. Initially, Lisa reports to the police that the driver had a green traffic signal, but later, out of remorse, changes her story. She confronts Maretti, who first pretends to have forgotten the details of the accident, and then reveals to her in anger that he does remember them, but believes he did nothing wrong, causing Lisa to pursue his firing from the company with passion. In collaboration with Monica's best friend, Emily, and cousin, Abigail, Lisa ultimately becomes involved in a wrongful death lawsuit against the Metropolitan Transit Authority, seeking the dismissal of the driver (who is revealed to have caused two previous accidents), as well as monetary damages, which would be awarded to the victim's next of kin, her cousin. Meanwhile, Lisa's life takes various turns, including a flirtation with her math teacher, Aaron Caije, her decision to lose her virginity to a classmate, Paul Hirsch, and various vehement debates with classmates about politics and terrorism.
Lisa and her actress mother have a rocky relationship, with sporadic fighting and Lisa expressing ambivalence toward her mother's boyfriend Ramon. An after-show dinner, attended by Lisa, her mother, Emily and Ramon, ends with Ramon making a remark perceived as anti-Semitic toward Emily. Ramon dies of a heart attack not long after. Lisa has sex with Caije, then later confronts Caije, telling him, in the presence of another teacher, that she has had an abortion. She expresses doubt about who the father was and mentions that there are two possibilities.
The lawsuit reaches a conclusion, with an award of $350,000, but the MTA refuses to fire Maretti, out of concern that it would inflame a labor dispute. Abigail claims the settlement offer, revealing the monetary settlement to have been her primary motivation; this causes Lisa to become very upset and disillusioned with the outcome of the case.
Lisa and her mother plan to attend an opera that Ramon and she were to see before his death. On the way, Lisa sees Maretti driving the same bus that had killed the pedestrian and there is a brief moment where the two see each other. During the opera performance, Lisa's accumulated emotion from the sequence of events bursts out and she and her mother affectionately reconnect, crying together and holding each other as the opera goes on.

Question: Why does Lisa change her story?
Answer: She feels guilty for lying to the police.
Alternative answer: Remorse.