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In the future year 202X, with Japan's economy in stagnation, experimental municipality Utopi-AI is inaugurated by Taira Kiyoshi by direction of Prime Minister Washida Tsuguaki. The city's "cabinet members" chosen by artificial intelligence (AI) unit Solon are all young enthusiasts, including "prime minister" Maki Aran, a 17-year-old high school student. Three months later, Sagawa Sachi, an admirer of Aran, relocates to Utopi-AI along with her family, and the experimental administration takes off. Aran immediately proposes to abolish the city council, and further vows to resign if and when his popular approval rate falls below 30%. The "teen regime" thus begins.

Maki Aran's reformative directives are opposed by former mayor Hosaka Shigeo and other senior city authorities, who argue for reinstatement of the city council. Aran refuses to cave in, and instead goes out to listen directly to the voices of the residents along with Sachi. At a traditional shopping district due for redevelopment he meets its opposer Suzuhara, who insists that "a scenery once lost can never be recovered." Moved by these words, Aran orders Solon to develop alternative plans and decides to hold a residents' referendum to determine the new plan for redevelopment. Aran's honest and straightforward approach to city administration gradually wins the heart of Kiyoshi, but a mystery develops over Aran's true motives.

Aran's reforms extend to streamlining of human labor by utilization of AI. Along this line, Sachi's mother Sagawa Tae, a middle school teacher, is designated as a candidate for resignation. The policy of eliminating unnecessary labor meets strong opposition from the residents, lowering Aran's approval rate to near 30%, the threshold for his resignation. Sachi, who has come to distrust Aran over the case of her mother, stumbles upon his secret, as does Kiyoshi, which is one that could cause damage to the Washida Cabinet.

Kiyoshi and Sachi are now aware that Aran's political aspiration was triggered by the death of his childhood friend Yuki, a victim of a scandal seven years earlier involving her father concerning a disguised monetary contribution to Prime Minister Washida. Suspecting that Aran is scheming to take revenge on him, Washida orders Kiyoshi to remove Aran from his post, and this puts Kiyoshi in a dilemma whether to undermine Aran or to defend Utopi-AI. Meanwhile, Sachi reveals to Kiyoshi her discovery that Aran has created a mysterious AI girl character in a hidden room in his residence.

A diary capable of triggering a scandal falls into Kiyoshi's hands.