The storyline? OK, deep breath, ready? The floating world of Cocoon seems like a nice enough place to live, heck it's clean enough, and all human needs look to be catered for.
But the citizenry are constantly in a state of fear. Fear of another invasion from the dreaded world of Pulse, fear of the Sanctum and their enforcing police Psicom, who conduct regular, somewhat sweeping 'Purges' of the population under the guise of weeding out those who contact the enemy Pulse.