Humanity has successfully colonized all the planets in the Solar System, with intentions to continue into new frontiers. Individuals live stable lives in these colonies, including young high school students living in 4th Tokyo on Mars. Two of these students are Iris Shirazaki and Mizuki Sera. Iris is finishing her last year of high school while studying to be a test pilot while her friend Mizuki also works as a mechanic on the aircraft. Her own brother, Kaito, oversees Mizuki and Iris as their homeroom teacher and program director.
Shido Itsuka, who used to be a normal high school student at Raizen High, meets the first Spirit, Tohka, and awakens his power to “seal the Spirit's power”. Though they may be feared and hated beings due to their immense power, Spirits were always alone and knew only combat and death. Shido, the only one in the world with the power to seal the Spirits' might, hurls himself into life-threatening dates with the beautiful Spirits in order to save them.
Sh?y? Hinata began playing volleyball after seeing the "small giants" who played the sport when he was in elementary school. He suffers a crushing defeat in his first and last tournament in middle school at the hands of his rival Tobio Kageyama. So, Hinata joins Kurasuno High School's volleyball team, vowing revenge against Kageyama. However, Kageyama is also on Kurasuno's team. The former rivals form a legendary combo with Hinata's mobility and Kageyama's precision ball-handling. Together, they take on the local tournaments and vow to meet Kurasuno's fated rival school in the nationals.
Arata Kaizaki is an unemployed, single 27-year-old who has just been cut off from his parents' financial support. Unable to find a job due to him leaving his previous employer within three months, he is stuck. One night after drinking with a high school friend, Arata meets Ryo Yoake, a man who offers Arata pills to turn him back into a 17-year old so he can redo his life. After agreeing to the experiment, Arata joins a high school class, and meets Chizuru Hishiro, a socially-awkward silent beauty who yearns to have friends. Through connecting with her and other classmates, Arata must find what he lacks to live a happy life in the real world within a one-year time limit.
Squid Girl has come to the land from the depths of the sea to conquer humanity for its pollution of the ocean. Unfortunately she ruins the first house she uses as an invasion base and has to work to pay for repairs. Of course, she can't overcome the Aizawa sisters who manage the house, so who knows whether she can subjugate humankind.