Ben Wolf, a resident of the small town of Trout, Idaho, has big plans for his senior year in high school until a blood test taken as part of a routine sports physical reveals that he has a deadly disease. The doctor tells him that his chances of survival are questionable even with treatment, but that without aggressive action he can expect to live no more than a year. Although the news is shocking, Ben has always known somehow that he would die young. His mother has a mental disorder, and Ben predicts that the added burden of his own illness will "break the fragile symmetry of [his family members'] lives." As he is eighteen, Ben invokes his right to keep his medical condition confidential, and, refusing treatment, resolves to take whatever time he has left and live life as normally and fully as possible.

Despite his small stature, Ben has always dreamed of playing football. With nothing to lose, he goes out for varsity, the team on which his brother Cody, eleven and a half months younger and a senior as well, is the star. Coach Banks is dubious when Ben first comes out for the team but is quickly won over by his tenacity and heart. Ben turns out to be an effective football player, "flattening guys half again [his] size" and playing with the intensity of one for whom there is no tomorrow.

With the same intrepid approach, Ben begins to pursue Dallas Suzuki, a tall, comely volleyball player who has long been "the focus of [his] lust and [his] undying love." To his surprise, Dallas invites Ben to meet her at a local hangout, and although he is initially disappointed to learn that she just wants to interview him for the school newspaper, he is astonished when, at the end of their meeting, she asks him to homecoming. Ben, elated, simultaneously experiences a deep sense of grief as it occurs to him that he is "gonna feel bad anytime [he] get[s] anything good, 'cause [he will have] to give it up" so soon. Dallas and Ben attend a Legion Hall dance on the heels of a football victory in which Ben, with Cody, plays a pivotal part. When Ben takes Dallas home afterwards, she invites him in and, at her instigation, they have sex. She then sends him off "like a stray cat," leaving Ben wondering if he and Dallas are "closer . . . or further away" and unsure of the implications if they are closer because in a year he will be gone.

Since learning about his impending death, Ben has been visited in his dreams by an enigmatic character named Hey-soos, who, as his name seems to indicate, is sort of a Christ figure, but not exactly. Hey-soos has the uncanny propensity of appearing in Ben's dreams when things are most complicated. He will not tell Ben what is or is not going to happen or if something is right or wrong; instead of giving him answers, he usually "brings [Ben] around to finding [his] own."

Educationally, Ben embarks on a quest for "truth," reading "subversive" books such as Loewen's Lies My Teacher Told Me and Haley's The Autobiography of Malcolm X. He challenges his current events teacher, Mr. Lambeer, an opinionated man who derides any viewpoint that does not agree with his own. Ben chooses to attempt to get a street named after Malcolm X as his class project, a formidable undertaking in a small town with a black population of zero. Mr. Lambeer balks at allowing Ben to pursue this topic, calling it "frivolous," but Ben is insistent. In the absence of his teacher's cooperation, Ben discusses Haley's book with Rudy McCoy, the reclusive "town drunk" whom he has befriended and with whom he has begun to establish a relationship based largely on their common interest in Malcolm X. At Ben's urging, Rudy has been making a real effort to stay sober. To Ben's surprise, when Rudy talks about the issues addressed in the autobiography, he "sounds like a college professor." Unlike Mr. Lambeer, Rudy encourages Ben to question what he is told because that is the way he will learn.

The homecoming game is a critical one, and Ben and Cody, again working in unison, execute a brilliant pass for a touchdown, bringing Trout the victory. After the game, Ben and Dallas go to the homecoming dance; the evening is perfect, and Ben thinks he is in love. Dallas invites him in again when he takes her home afterward but they do not sleep together. Instead, Dallas confesses that she initiated sex the last time because she was testing herself to see if she had been "ruined," rendered incapable of having normal physical relations with anyone else, by an uncle who had molested her.  Ben begins going over to Dallas's house quite frequently after this, spending time with her and her little brother Joe Henry. As their relationship grows deeper, Ben is increasingly disturbed because he knows Dallas will get hurt when he dies but he has not been able to find the courage to tell her the truth about his condition. One evening, Dallas shares with him another revelation: Joe Henry is actually her son, the result of her Uncle's molestation. Dallas says that she hates secrets because "they'll kill you." Although Ben recognizes that this is the time to reveal his own secret to Dallas in return, he is, as he says, "a coward" and misses his chance.

When Ben goes to visit Rudy in early December, he finds that his friend, who had been doing so well, is completely intoxicated. In utter despair, Rudy confesses that he had been a Catholic priest and had molested a child in the past; he has promised himself that if he ever feels that fatal urge again, he will end his own life. Rudy then admits that he is attracted to Ben, and Ben, confused, leaves precipitously; the "subtleties and paradoxes" of life to which he has been exposed in the past weeks are too much to handle. After some reflection, however, and a meeting with Hey-soos, Ben decides that by admitting the truth Rudy has already prevented "the worst thing" from happening. Using his best judgment, Ben tells Rudy that he will stick with him and that they will take one day at a time. Rudy tiredly agrees but makes no guarantees.

Ben now fully understands that he has to tell the truth about his condition to those he loves, but he still does not know how. He has begun to feel the effects of his illness and is terrified at what lies ahead. He asks Hey-soos why he didn't tell him in the very beginning how foolish it would be to keep it a secret. Hey-soos replies that although he did hint at it, in the end it was up to Ben to decide because he has free will. Ben first reveals his secret to Rudy on Christmas Eve, but he does not stay to get a reaction. In early January, he tells Coach, who has had experience with unexpected death; after his initial shock and incredulity, Coach promises to support Ben in whatever way he can. Ben goes next to see Dallas, who is at first completely sympathetic, but when it dawns on her that Ben knew that he was dying when he started seeing her and didn't tell her the truth even when she trusted him with her own secrets, she is overcome with a feeling of betrayal and tells him to leave. Mid-January is miserable for Ben; Dallas will no longer speak to him, and he increasingly feels poorly. Cody perceives his distress and Ben tells him the truth as well. Grieved, Cody understands why Ben chose to handle the news of his illness the way he did, but he is sorry that Ben did not feel he could tell him about his situation at the very beginning.

Things deteriorate quickly for Ben from this point on. On his next visit, he finds that Rudy has indeed taken his own life, leaving a note apologizing for taking the "cowardly" way out and thanking Ben for his friendship. Ben finally tells his parents that he is going to die, regretfully leaving his father to handle his own grief on top of that of his unstable mother's. By mid-February, Ben has begun missing school because of his illness, and when he is able to be present, he engages in heated arguments with Mr. Lambeer about history, patriotism, bigotry, and truth. At the end of one particularly contentious confrontation, Ben blurts out the fact that he is dying to the entire class; now everyone knows what is in store for him. Ben desperately wants to make it to graduation but senses that "it ain't gonna happen." In the midst of his grief, Cody signs at Boise State to play football, and Ben knows his brother will be all right. On the last day of his life, Dallas comes to see Ben, and she lies next to him with her head on his chest as he slips into unconsciousness.

Cody reads the speech that Ben would have delivered at graduation had he lived that long. Ben has written about the last year of his life, sharing the lessons learned through his experiences. Ben encourages his class to be unafraid to take risks and notes that he made his best decisions when he lived life to the fullest while remembering that his actions "had consequences not only for [himself] but for everyone [he] touched, forever." Most of all, he urges them to always tell the truth. The story ends with Cody and Dallas running up the reservoir road with Joe Henry in a jogger's stroller. Both will be attending Boise State in the fall, and Cody offers to help Dallas care for her son. The memory of Ben lives on strongly within them both.