Tistou is an eight-year-old boy from a family of immense wealth; his father owns a successful armament factory supplying weapons to multiple countries. Struggling with conventional schooling, Tistou consistently succumbs to sleep during lessons, leading to his dismissal after only three days. His parents opt for experiential education, learning through direct observation of real life.

Assigned to study gardening under the family gardener, Mr. Mustache, Tistou discovers he possesses green thumbs. Anything he touches spontaneously erupts in lush plants and flowers, even on surfaces like stone or metal without soil or seeds. He also communicates with animals, particularly his pony, Gymnastique.

Guided by Mr. Turnbull, the stern factory manager, Tistou tours his hometown, Mirepoil, visiting its prison, slums, and hospital. At each location, he uses his power: flowers and plants transform the grim prison walls, impoverished slums, and sterile hospital rooms. Observing this, he concludes that "flowers prevent evil things from happening," believing beauty can counter despair and suffering.

Learning about war as the "greatest and most horrible disorder," Tistou visits his father's factory and discovers it supplies weapons to opposing nations in conflict. He uses his power on the weapons. When deployed, the tanks and cannons sprout flowers instead of firing bullets, effectively halting the war.

Tistou later departs quietly one night. Mr. Mustache reveals Tistou was not an ordinary child but an angelic being; the ending suggests his mission culminated in a departure from the human world.

Throughout, Tistou exhibits inherent kindness, wide-eyed curiosity, and unwavering empathy. He has blonde hair, blue eyes, and typically wears a sailor-style outfit. As an all-loving hero, he uses his miraculous gift solely to alleviate suffering and create beauty, impacting prisons, hospitals, slums, and the machinery of war.