This is the episode that exists purely to let you breathe. The TV series ran on autumn deadlines: the old Uchihama Academy building counting down to demolition, the last General Club Festival, the student executive committee dumping ghost rumors and sleeping sickness reports on the Astronomy Club, and then Kaori Sasaki gone in an instant and a girl named Yui Furukawa standing there claiming she came from the future. After thirteen episodes of that math, a bonus disc about a lost summer vacation is not filler, it is restitution. Watching Sou and the club occupy time that has no ticking clock attached to it hits differently once you know what October did to them. It shipped only to people who pre-ordered all six Blu-rays, which is exactly the crowd that needed it most.

Yui Furukawa comes to and finds that the date is July 13, a starting point she has never lived through before. Unfamiliar as it is, she decides to spend summer vacation with her fellow Astronomy Club members at the villa owned by Nagisa Hanamiya's family. The area has a long past: Nagisa's great-grandmother was a celebrated scientist who developed technology at a military manufacturing plant nearby. A beach day is cut short by a sudden storm, so the club takes refuge in the abandoned factory, where they eventually come upon a hidden bunker below ground. What waits there may tie into a lost future. This romance special was made as a thirteenth episode, given as a bonus Blu-ray disc to buyers who pre-ordered all six volumes.