Kousuke and Natsume have been friends since childhood, but as time goes on, the relationship between the two sixth graders seems strained as they keep avoiding each other. One day during the summer vacation, they visit a housing complex that is scheduled to be demolished. Having grown up there, the place holds a lot of memories, but while playing, they suddenly get caught up in a mysterious phenomenon. When they regain consciousness, they see an entire ocean before them as the housing complex has drifted into a mysterious sea with Kousuke, Natsume, and others in it. Will they be able to return to their previous world? A summer farewell journey begins...

Second half of .

takes place during a sweltering midsummer day when Ozu, the protagonist's "terrible friend," accidentally drowns the only remote control for the air conditioning unit in their apartment. While they discuss their predicament with Akashi, the protagonist's raven-haired love interest, a sloppily-dressed time-travelling student arrives from 25 years in the future, prompting the protagonist to borrow his time machine in an effort to snatch the remote control from the past before it breaks.

Fifth season of .