The conquest of the Universe! This is roughly the main storyline of "Once Upon a Time ... Space". In the future, our well-known friends, Maëstro, Pierrot, Petit Gros, accompanied by the newcomers Psi (Mercedes by her real name) and the Metro robot, will go from adventure to adventure, from planet to planet, it will be a question of prehistory, ecology, new energy resources, etc. They will notably be confronted with the terrible confederation of Cassiopeia, led by the ambitious Teigneux and his henchman the Dwarf.