In 1991 in New York City, Alyssa "Ally" Craig is waiting with her mother for the subway when they are mugged by two young men who shoot her mother after boarding the train.
Ten years later, Ally is a student at New York University and lives with her father, Neil, a New York Police Department detective. Tyler Hawkins audits classes at NYU and works at the university bookstore. He has a strained relationship with his businessman father, Charles, because his older brother, Michael, committed suicide years before. Charles ignores his youngest child, Caroline, of whom Tyler is protective.
One night with his roommate, Aidan, Tyler gets involved in somebody else's fight and is arrested by Neil. Aiden calls Charles to bail Tyler out, but he does not stick around to have a conversation with his father. Aidan sees Neil dropping Ally off, realizing that she is his daughter. He approaches Tyler with the idea to get back at the detective by persuading him to sleep with and dump Ally. Tyler and Ally go to dinner, kiss at the end of the night, and continue seeing one another. While at Tyler's apartment, Aidan convinces the pair to go to a party, after which Ally is very drunk and ends up crashing there. The following day she and her father argue. Neil slaps her and Ally flees to Tyler's apartment.
Caroline, a budding artist, is featured in an art show and Tyler asks his father to attend the show. Tyler confronts him in a board room filled with people, which causes his father to explode. Neil's partner recognizes Tyler with Ally on a train, so Neil breaks into Tyler's apartment and confronts him. Tyler provokes Neil by confessing to Aidan's plan and his initial reason for meeting Ally, which forces Tyler to confess to Ally. She leaves and returns home. Aidan visits Ally at her father's home to explain that he is to blame and Tyler is in love with her.
Caroline is bullied by a classmates at a birthday party where they cut her hair off. Ally and Aidan visit Tyler's mother's apartment where Caroline is sobbing. Tyler accompanies his sister back to school and when her classmates tease her for her new haircut, Tyler turns violent and ends up in jail. Charles is impressed that Tyler stood up for his sister, and they connect. Charles asks Tyler to meet with the lawyers at his office.
Tyler spends the night with Ally and they reveal they love each other after making love. Charles takes Caroline to school. He calls Tyler to let him know this and tell him he'll be late. Tyler is happy his father is spending time with Caroline. He tells Charles he will wait in his office, He sees on Charles's computer, a slideshow of pictures of Tyler, Michael and Caroline when they were younger.
After Charles drops Caroline off at school, she sits in her classroom, where the teacher writes the date on the blackboard as September 11, 2001. Tyler looks out the window of his father's office which is revealed to be located on the 101st floor of the North Tower of the World Trade Center. Once the 9/11 terrorist attacks begin, the rest of the family, Aidan and Ally look at the towers before the camera pans over the rubble, showing Tyler's diary. In a voice-over of his diary, Tyler reveals to Michael that he loves him, and he forgives him for killing himself. Tyler is buried next to Michael.
Some time later, Caroline and Charles seem to have a healthy father-daughter relationship. Aidan, who has since gotten a tattoo of Tyler's name on his arm, is working hard in school and Ally gets on the subway at the same spot where her mother was killed .
Output: Why did Tyler act violent towards Caroline's classmates?