Yui Furukawa wakes up to a new day, July 13th, which she has never experienced. Despite the uncertainty, she decides to spend her summer vacation with the Astronomy Club members at Nagisa Hanamiya's ancestral villa, which has a rich history thanks to Nagisa's esteemed great-grandmother's research and technology development at a nearby military manufacturing plant. Their beach plans are thwarted by a sudden storm, and the club seeks shelter at an unused factory, where they stumble upon a concealed underground bunker. The contents of which may reveal alarming connections to a distant tomorrow.