In modern Europe, the world has fallen into chaos due to a sudden anomaly.

Doppelgangers. Eerie beings capable of mimicking any appearance and even voices, their synchronization rate increasing as time passes.

At first, they were a novelty, and many people, captivated by their communicative intelligence, continuously attempted to converse with them.

That is, until the former Director of the Quarantine Authority was killed by a doppelganger he had been keeping publicly, which had mimicked him perfectly.

According to a statement made by that doppelganger before it was neutralized, their goal is 'Perfect Replacement.'

Faced with an unknown threat whose origin and true form remain a mystery, humans did not hesitate to conduct biological experiments.

And finally, one fact was uncovered: doppelgangers cannot hear the radio. More precisely, they cannot translate those frequencies into language.

We still do not know how they hear the world or what they hear.

Since that day, radios have been installed at the entrances of every building.

Like a gate that separates humans from the non-human.