Xenius takes you to the rooftops of Paris. Presenters Dörthe Eickelberg and Pierre Girard meet biologist Yohan Hubert in his vertical garden. With the world's population constantly increasing, this pioneer dreams of cleaning up the atmosphere and lowering the temperature in cities using plants, while producing food. However, the surface area of ​​green roofs is too limited to allow for real urban agriculture. To develop it, indoor farms could be a solution. Are green cities a utopia or a real prospect for the future?