Narrated in the present tense by an unnamed omniscient narrator, Loser opens with a description of Donald Zinkoff, the novel's protagonist. Zinkoff is largely ignored by other people in his working-class town. Most people don't know his name. He has no friends or enemies. To most people, he is simply an ambient presence, another kid in the neighborhood, occasionally drawing attention to himself by doing something clumsy or awkward.
When he begins first grade, Zinkoff is oblivious to his flaws and weaknesses. He remains positive and enthusiastic when others might be embarrassed. Zinkoff possesses an inner joy that baffles most people. He’s not smart or athletic, and the other kids know it. He is so excited to start school that he can barely stop laughing, which his classmates think is weird. Miss Meeks, his teacher, worries that he is a troublemaker. But she finds Zinkoff is polite and obliging whenever she suggests he change his behavior.
At home, Zinkoff is supported and encouraged by his mother and his father. His father is a mailman—a noble, heroic profession in Zinkoff's eyes. His mother gives birth to a daughter named Polly. Around the same time, Zinkoff tries to befriend his neighbor Andrew. But Andrew is reluctant to be friends. Zinkoff's attempt to woo this new friend by baking a sheet-pan–sized snickerdoodle cookie ends in disaster when the cookie breaks and falls on Andrew's living-room carpet.
In the second grade, Zinkoff's lack of self-awareness tests the patience of his new teacher. She insults his handwriting, calling it atrocious, but he misunderstands, assuming the word is a compliment. Eventually she erupts in anger when Zinkoff accidentally vomits on her prized chalkboard eraser. She sends him out of the classroom and tells him never to return to school. He believes she means this sincerely, but returns the next day, early to class, once the principal sorts out the confusion.
Once Zinkoff reaches the fourth grade, kids who never acknowledged his difference now notice every socially awkward move Zinkoff makes. During Field Day, Zinkoff is too uncoordinated to compete in the one-leg race, which he loses for his entire team. As a consequence, the other students call him "Loser," a nickname that sticks with him into fifth grade. The next year Zinkoff trains in advance of Field Day. However, he skips the day, too ashamed to take part.
As Zinkoff enters middle school, he transitions from being the butt of all jokes to being completely ignored. Isolated and alone, he struggles to deal with his solitary way of life.
One day there is a massive snowstorm that blankets his town. At the same time, a little girl named Claudia goes missing. Zinkoff goes off on his own in search of her, looking in the dark alleys behind her home. Searching for more than seven hours, Zinkoff experiences hallucinations, falling asleep on his feet. He braves falling temperatures and almost dies of exposure to the cold, but remains fixated on finding the little girl. Eventually he is rescued and brought home. After sleeping for thirteen hours, Zinkoff finds out that Claudia was found not long after she went missing; she had merely wandered into a neighbor's garage. Zinkoff wonders why he saw the police and emergency vehicle lights still illuminated. His mother tells him they were looking for him.
While Zinkoff's failed search marks another of his misadventures, it also demonstrates his remarkable character and courage. The adults involved in the search see Zinkoff as a hero. The novel ends with a chapter from the perspective of some other boys at Zinkoff's school. From a distance they tease Zinkoff and talk about what a loser he is. One boy explains what happened when Claudia went missing, leading another boy to wonder what it would be like to be so cold for seven hours. The boys stop talking to call a game of football, dividing into teams. Zinkoff is the only player not to be picked. The teams are already even, but Zinkoff refuses to accept the rejection and walk away, perhaps because he is oblivious. Eventually one of the team captains picks him and the game begins.
