You can’t see them. You can’t hear them. And you never know exactly where they are. The life of submarines, these masters of discretion and guardians of our oceans, is secret and fascinating. Take an unprecedented voyage inside one of the French navy's most legendary vessels, the Rubis, before resurfacing to visit the Cherbourg shipyard where the next nuclear submarine, the Barracuda, is being built. From technological prowess to human exploits, the film submerges us in a timeless amphibian adventure.