In the late 21st century, mysterious structures known as "Nightmare Domes" began to appear all over the world, causing a global crisis. The only thing that could stop them was the "Swan", an idol's singing voice. In order to counter this threat, the United Nations has created the special divison "IDUN", or "Idols Disciplined to Unpurified Nightmare". Six young girls from various corners of the world (Seika, Tamamo, Rose, Isabella, Katya and Mo Xi) have been selected, taking on the various challenges along the way. Whenever the peace is disturbed, it is up to them (and you, the audience) to fight together. (Source: VTuber Wiki)