When eight-year-old Tistou is sent home from school after being unable to stop falling asleep at lessons, his father decides that he will continue his education by learning from observation of real life, and where better to start than in the garden!

With the gardener Mr. Moustache, Tistou discovers he has a remarkable gift—the green thumbs: everything he touches grows into beautiful plants. His next lesson is with the stern Mr. Turnbull, manager of his father's armament factory who takes Tistou to visit the town to learn about "order."

Over the course of the next few days Tistou learns many new things as he visits the town prison, the slums, and the hospital, and everywhere he tries to change the world into a better place with his new extraordinary power. Finally, he learns about war, and upon visiting his father's factory, he finds out that they supply the two countries at war with their weapons...