Narrated by the book's author, Roald Dahl, Boy: Tales of Childhood begins with Dahl explaining his ancestry. His father, Harald, while living in Norway in the 1800s, has his arm unnecessarily amputated by a drunk country doctor. Harald moves to France and starts a shipbrokers business with another Norwegian, moving it to a port in Wales. After Harald's first wife, Marie, dies following the birth of their second child, Harald marries Sofie, a fellow Norwegian. They have four more children, one of whom is Roald Dahl.
When Dahl is three, his sister Astri dies of appendicitis—a deadly illness in an era when penicillin wasn't available. Harald, overcome with grief, soon loses any interest in living and dies of pneumonia. Sofie gives birth to another child two months later. Instead of moving back to Norway, she stays in Wales to fulfill her husband's wish that his children go to English schools, which he considers the best in the world.
As an elementary-school student, Dahl develops a love of candy but a dislike of Mrs. Pratchett, the unpleasant woman who runs the candy shop. He puts a dead mouse in her jar of gobstoppers, a prank he and his friends are punished for by their school Headmaster, Mr. Coombes. With Pratchett egging him on, Coombes strikes the boys' bottoms with a rattan cane. The incident enrages Sofie, who decides she will send Dahl to an actual English school, not a Welsh one.
After a summer visiting family in Norway, Dahl begins living in the dormitories at St. Peter's, a boarding school in Weston-super-Mare, a town just over the border in England. He is terrified by the Headmaster and the angry Matron in charge of the boys in the dorms, a woman who pours soap flakes down the open throat of a boy who is snoring. Dahl fakes ill to return home, but has to go back to boarding school when his doctor knows he is merely homesick.
At thirteen, Dahl starts attending a private big school (high school) called Repton Prep. He has another violent Headmaster at Repton. In this case, the man is a clergyman. Dahl finds it ironic and hypocritical that he preaches love and forgiveness as someone who viciously beats young boys with a cane. Dahl comments that the man will eventually become Archbishop of Canterbury, the top religious authority in Britain. On television, Dahl and half the world watch the man crown Elizabeth II as Queen.
The other terrors at Repton are the Boazers, older student prefects put in positions of authority over the younger boys. Any infraction against their rules or failure to meet unreasonable standards of perfection results in canings. While at Repton, Dahl garners attention as captain of various teams, a position of high regard at the school, but he isn't welcomed into the hierarchy of school officials and Boazers because they can sense his indifference to authority.
Dahl finishes high school at seventeen and leaves Repton on his secret motorcycle, happy to leave the place behind. Rather than go to university, Dahl takes a sought-after job with Shell Oil. He undergoes two years of training in England before being sent on assignment abroad. He is displeased with the idea of going to Egypt, wanting jungles and exotic animals. His boss sends him to east Africa instead, and Dahl goes happily, learning Swahili while there. His career is put on hold, however, when WWII breaks out. Dahl joins the Royal Air Force and becomes a fighter pilot.
The book ends with Dahl commenting on how there are many war stories, and other stories of adulthood, but those belong to a different book. He says they have nothing to do with the lasting memories from his childhood.
