The youngest son of famous explorer Jacques Cousteau gets on board with marine biologist Enric Sala in Marseille to explore the Mediterranean seabed and compare the evolution of its flora and fauna over the last six decades. On board the turbo-propelled oceanographic ship Alcyone, the explorers set off with a National Geographic team to visit four Mediterranean marine reserves. The wealth and biodiversity that existed less than a century ago in these areas have decreased alarmingly. This is why marine protected areas are instrumental in the regeneration of the Mediterranean's fauna and ecosystem.