It's been about a year since he returned from Ogasawara.

It's the start of summer. Aiuchi Shuuta, now a third year at the academy, has lazily watched the seasons pass-- his carefree classmates, his peaceful day-to-day life... and his love who hasn't returned, Arimura Romi.
He simply vacantly watched and continued to wait.

And to him comes a letter from the head of the Comet Organisation, Miko.

Once again, along with his comrades and the rest of the world, he gets caught up in their big plans regarding the comet. The final summer was about to begin.

[Adapted from the official page]