In the town of Saima, when summer is about to begin, a group of six high school students intend to live a secret adventure by spending seven carefree days in a ruined mining facility.
As the world is threatened by the sudden appearance of a mysterious alien mothership, best friends Koyama Kadode and Nakagawa “Ontan” Oran carry on about their high school life. But as they grow up, they face existential questions, learning adulthood’s complexities and that the true threat may not be from above.
Inspired by his tennis champion father, Ryoma is determined to take his game to the next level and train in the US. But in the middle of an intense tennis battle to save his classmate Sakuno from dangerous villains, they somehow find themselves transported back to the past, just as Ryoma’s father is about to undertake his final championship match at the US Open – when he was mysteriously forced to retire.
