Thirteen to fourteen years ago, creatures known as 'Noise' started attacking the planet. The United Nations General Assembly has observed these creatures as a 'special disaster', but have been unable to do anything to really combat them without sacrificing the lives of the people of the world.
This is where the relics known as 'Symphogear' come in.
Created from the "Sakurai" theory, Symphogear is the name of a unique, special counter-measure armor that was created through the phonic gain of items known as 'Relics'. Symphogear is the only known weapon in the world capable of destroying the Noise without endangering the humans attacking them.
This themelisting for Senki Zesshou Symphogear picks up at the end of episode six, season one of the anime. The corresponding manga location would also be at the start of chapter seven.