In Canada in the spring of 1968, the cob, the name for an adult male swan, and the pen, the name for an adult female swan, both members of the swan species trumpeter swan build their summer nest on a small island in a pond. Sam Beaver, an 11-year-old boy on a camping trip with his father, observes them, and saves the female from being attacked by a fox. The swans begin to trust him, and Sam gets to see their five eggs hatch. All of the cygnets chirp at Sam in greeting, except for the youngest, who can make no sound and pulls his shoelace instead.
The cygnets' parents are increasingly concerned about their youngest son, Louis, who turns out to be mute. They worry that when he grows up, he will not be able to find a mate if he cannot trumpet like all the other swans. Louis's father promises to find a way for him to communicate. At the end of summer, the swan family flies to the winter refuge, Red Rock Lakes in Montana. Louis decides he should learn to read and write in order to communicate, and flies away from the refuge. Because Sam Beaver lives nearby, he happily takes his swan friend to school with him the next morning. Louis turns out to be a natural at reading and writing, and Sam buys him a portable blackboard and chalk so he can communicate.
Unfortunately, because the other swans cannot read, Louis is still lonely when he returns to the Red Rock Lakes. He falls in love with a young swan, Serena, but cannot attract her attention. Louis's father flies to a music store in Billings, Montana, crashes through the window, and steals a brass trumpet on a cord to give to his son. Louis feels guilty about his father's theft, but accepts the instrument. Serena has migrated north, so Louis returns to Sam's ranch. Sam suggests that Louis get a job so he can pay the store for the trumpet and the damaged window, and Louis finds a position as camp bugler at Camp Kookooskoos, the boys' camp Sam attends.
Louis plays taps, reveille, and mess call, and composes a love song for Serena. He convinces Sam to split one of his webbed feet with a razor blade, making "fingers," so he can play more notes. He also rescues Applegate Skinner, an unpopular camper who nearly drowns. At the end of summer, Louis receives the Lifesaving Medal, a waterproof moneybag, and $100. Sam suggests that Louis get a job with the Swan Boats in Boston. He flies across country and becomes an instant success, with a salary of $100 per week. He even stays in the Ritz Hotel.
A Philadelphia nightclub offers Louis a higher salary, $500 per week. He leaves Boston and takes up residence at the Philadelphia Zoo. The zookeeper promises that Louis will not be pinioned (have a wing tip cut off to prevent escape) like all the other swans at the zoo. One windy night, Serena, blown off course, falls into Bird Lake. Louis serenades her with his trumpet, and she finally notices him. But when the zookeepers spot Serena, they try to clip her wings, and Louis attacks them. He convinces the Head Man to postpone the operation for a short while, and sends a telegram to Sam, asking for help. Sam goes to Philadelphia and strikes a deal with the Head Man: in every litter of cygnets there is always one that needs special care and protection, and if Louis is willing to donate an occasional cygnet to the zoo, the Head Man will let Louis and Serena go free.
Louis and Serena fly back to the Red Rock Lakes. Louis writes an apology on his slate and gives it and the moneybag to his father, who flies back to the music store in Billings. Afraid that the swan will destroy another window, the storekeeper shoots the old cob in the shoulder. The cob recovers and flies back to the Red Rock Lakes. In the country, when Sam is about 20 years old, he is again camping in Canada, and hears Louis playing taps to his children. He writes in his journal: Tonight I heard Louis's horn. My father heard it, too. The wind was right, and I could hear the notes of taps, just as darkness fell. There is nothing in all the world I like better than the trumpet of the swan.