The script opens on the conclusion of a graveside service for a robust and wealthy young man (Franklin Smith) who died after being hit by a car which was ostensibly driven by a 19 year old Cambodian immigrant, Chay Chouan. Chay has been jailed and charged with vehicular homicide.

Franklin's parents, (Robert and Ingrid Smith) and his fourteen year-old brotheer Henry, (14), leave the cemetary and go to their coastal mansion just north of Boston, Massachusets. The year is 1986. The family is "old money Massachusets" and has never had trouble such as this. Louisa Smith (16), a sister to the deceased Franklin, is so distraught that she did not go to the funeral and refuses to come out of her room or interact with the family. The whole family is distraught.

A month after the funeral, Henry saves a badly beaten black and starving dog from drowning. He begs, and finally is granted the privilege of keeping her. He names her "BD" for Black Dog. That night, Henry recalls Franklin’s dying word: “Katahdin” and decides that he will climb it.

At his school, elite WHITTIER-LONGFELLOW PREP SCHOOL, Henry tells his best friend, Sanborn about his decision to climb Katahdin. Sanborn insists he will go. Henry’s repeated “You’ll never make it,” causes a fight between the two friends. Sanborn accuses Henry of sounding “just like your brother.”

At the sentencing hearing for Chay, who has confessed and waived his right to a trial, witnesses testify about Chay’s leaving the scene of the accident in order to get help. Examination of witnesses reveals bullying of Chay. The judge rules that the tragedy was entirely an unintentional accident. Chay gets probation and suspension of driving privileges.

Henry sneaks out of the house in the early morning, intent on hitchhiking with BD and with Sanborn to climb Katahdin. Not until it is nearly evening does anyone stop. The vehicle is a pickup driven by Chay. Henry grows angry. It is only Sanborn’s pleading and the late hour that keeps him in the truck. Strangely, BD tries at every opportunity to show Chay her love.

A road trip to Katahdin ensues in which the three boys and the dog have several confrontations with each other, with violent and drunken outside parties, and with police of whom Chay is very afraid.

In one of the encounters, Henry learns that it was his sister, Louisa, who was actually driving the vehicle that hit and killed Franklin. He also learns that Chay and Louisa have been having a secret romance.

The story ends with Henry getting wounded by the violent drunks whom they met earlier and with Chay and Louisa coming clean about their relationship and about the accident.