Toumei Answer

The story where one averts one's eyes.

Background
(Nicovideo broadcast is not available because the PV was originally meant to be exclusive to the album Mekakucity Days, and the person who uploaded the video onto Nico was simply posing as Jin.)

This is the story about how Shintarou became a hikikomori (recluse/shut-in).

Shintarou was a student with straight A's. When in school he met Ayano who he didn't want to get close to but ended up doing so anyway. Ayano had bad grades and called herself stupid. They then enter their first year in high school together. One day Shintarou sees Ayano crying but doesn't know what to do and retreats. The next day he finds out she died when people bring in flowers for her. It is revealed she committed suicide for an unknown reason. Shintarou goes up to the roof of the school and finds one of the paper airplanes that she sent him. The paper airplane was a test Ayano got 100 on for (most likely) the very first time. Her death put him in so much shock that he wanted to escape reality and enclose himself in a digital world which he did by locking himself in his room and becoming a NEET.