26-year-old car mechanic Hibiki Mamiya lives with his high school sweetheart of the same age, Kurumi Ogasawara. Life together feels so natural and filled with the warmth of each other's love. It was a happiness they expected would last a lifetime.

Hibiki is secretly preparing his proposal to Kurumi when misfortune strikes. A tunnel collapses, trapping him for four days. Barely making it out alive, he discovers a world completely changed. There is absolutely no one around, communication signals are dead, the transportation network is a wreck, and the only information available is the emergency evacuation order being repeated on the radio. Everywhere Hibiki goes, he sees bloodstains.

Just what happened to this town?!