For nearly 50 years, the botanist Francis Hallé has surveyed and studied the great old-growth tropical forests. We follow him into the forests of Nyonie in Gabon and discover the strategies of plants with him. Thanks to unique inventions like the “canopy raft”, a giant raft attached to a hot-air balloon, the botanist takes us among the tree tops, veritable treasures of biodiversity. His perseverance and ingenuity push back the limits of established knowledge: his major discovery remains his new system of classification that could revolutionize botany. His models, as well as the body of his work, give us the keys to understanding and admiring the genius of plants, to better grasp their wealth and, most of all, their fragility, thereby awakening awareness to the importance of old-growth forests and biodiversity for the planet’s future.