In Neo-Seoul, 2031, black envelopes sealed with a crimson X appear before impossible deaths. No one knows who sends them. No one survives after receiving one. As panic spreads through the city, a relentless detective uncovers a terrifying truth: the envelopes are not threats—they are invitations. And somewhere in the shadows, a silent figure known only as HAR is already deciding who receives the next delivery. When the letter arrives, the story ends.