Halley Marie Martin (Mandy Moore) is a seventeen-year-old high school student who is disillusioned with love after seeing many dysfunctional relationships around her. Her parents are now divorced and her father, Len Martin (Peter Gallagher), a radio talk show host, has a new young girlfriend that the entire family despises; mainly because she's the reason for the divorce. Her mother, Lydia (Allison Janney), is now always alone while her sister, Ashley, is so overwhelmed by her upcoming wedding that she barely exists in the house anymore. On top of that, the shallowness of all the girls and guys at her school convinces Halley that finding true love is impossible. When tragedy strikes, the life of her best friend changes forever - and suddenly Halley is drawn closer to a classmate named Macon.
When Halley walks in on her best friend Scarlett having sex with her boyfriend, the high school soccer champ, Michael Sherwood, Halley tries to warn her of the complications that lie ahead. Ignoring her advice, Scarlett embarks on a summer fling with Michael. In the meantime, Halley must deal with Ashley and the rest of the Martins must deal with her soon-to-be in-laws; Louis Worscher's overbearing Southern parents, who still have a maid, who just happens to be African-American.
In the meantime, Halley runs into classmate, Macon Forrester, a slacker who never shows up for biology and is more interested in having fun than school work. He is Michael Sherwood's best friend.
A few weeks later, Michael dies of a heart defect on the soccer field while Halley, Scarlett and other classmates watch helplessly. While Scarlett does her best to look beautiful for Michael at his funeral, Macon gives a moving speech about his friend. Struggling with Michael's death, Scarlett, at age sixteen, soon learns that she is pregnant with his child. The two girls hatch an escape plan that is ultimately foiled when Scarlett's mother (also Lydia's best friend) catches Halley trying to pull the car into the garage. With Halley at her side, Scarlett reveals the pregnancy to her mother.
Meanwhile, things go awry when Halley finds Ashley drunk on the family's front porch. Finding a male stripper's thong around her neck, Halley, who is recovering from a broken arm she got after a car accident with Macon on New Year's Eve, witnesses the break-up of her sister's engagement. She also learns that her mother sneaks out once a week to have sex with a man she met a couple of weeks previous, Steve. After another make-up, Lewis proposes to Ashley again, this time at a crowded airport; she says yes. Halley's father, Len Martin marries his mistress, Lorna, in a beach-side ceremony, with guests from all over the world...or "within his radio frequency" as Halley puts it.
In the end, Ashley and Lewis are married, as Lewis puts his foot down on the boundaries between his marriage to Ashley and his parents. Scarlett gives birth to a baby boy that she names Michael, and Halley and Macon agree to pursue their relationship again.