Nobuo Koyama, father to Kadode Koyama and a manga editor, vanishes without warning during the alien mothership's arrival over Tokyo on August 31, 2010 (8/31). His disappearance fractures his family, creating lasting emotional tension with his daughter and wife.

Years after 8/31, Nobuo reemerges within the dystopian "Never-ending Summer Vacation" era. Here, Tokyo's destruction by the mothership has triggered global societal collapse. Japan splinters into warring factions, lethal "asterisk" energy spheres decimate populations, and autonomous weapons patrol the ruins. In this shattered world, Nobuo relentlessly searches for Kadode, though evidence hints she may have perished alongside Ouran during the initial cataclysm.

Navigating this perilous landscape, Nobuo encounters Makoto and Futaba. They possess alien technology enabling inter-dimensional travel and offer him a chance to shift his consciousness to an alternate timeline twelve years before 8/31, where the alien invasion never occurs. Accepting their offer, he arrives in this new reality. His actions there alter events, ensuring his family remains whole and sparing Kadode from the original timeline's traumas. This intervention secures a peaceful existence for Kadode and Ouran, contrasting their grim fates elsewhere.

Nobuo's journey intertwines with the series' exploration of causality and sacrifice, underscored by the revelation that Ouran herself initiated timeline shifts to prevent Kadode's descent into vigilantism and suicide in another reality. Nobuo’s final act of timeline alteration responds directly to these cascading events, positioning him as an inadvertent architect of stability amidst multiversal chaos.