Black Cat Cafe is located in the fifth ward of the Imperial Capital. Having lost their purpose after the war ended a few years prior, advanced mechanical dolls known as Automata have been restored and now work at the cafe wearing elegant kimonos. While they adjust to the peaceful world, events of the past continue to haunt the Automata, preventing them from letting go of the responsibility they once held. Among the Automata working at Black Cat Cafe is the recently restored Haizakura, who has lost her memories during a reset. Her identity and role in this unfamiliar world are a mystery to her, but one thing is certain—she loves to sing. With never-ending curiosity and beautiful songs to guide them, Haizakura and the Automata search for their new purpose together, supporting each other no matter how rigorous that endeavor might be.

These are witches who defend everyone's smiles and fight enemies through the healing power of music. These witches, opposite of those in the defense fleet, are idols known as the Music Squadron.

After surviving the brutal challenges of Idofront, Riko now possesses a White Whistle, allowing her to descend into the Abyss's sixth layer—The Capital of the Unreturned. Alongside Reg and Nanachi, Riko begins to explore the uncharted domain, where the ruins of the promised Golden City are located. As the trio starts to adapt to the harsh environment, they soon encounter dangerous creatures and treacherous landscapes. Their expedition leads them to a village inhabited by strange beings known as "hollows." Despite the creeping sense of unease that welcomes them, the three venture onward to uncover the mysteries of the settlement and long-lost legacies of the forgotten adventurers who once descended into the horrors of the unexplored Abyss.

As technology advanced, humanity gradually colonized the entire solar system, occupying planetary facilities from Mercury to Pluto. This results in a large-scale athletic tournament, the "Daiundoukai," where several aspiring female athletes from all around the solar system compete. The winner of the tournament is crowned the "Cosmo Beauty," obtaining a queenlike status. When outgoing potato farmer Kanata Akehoshi was a child, she witnessed a meteor-like object falling from the sky, crashing in a nearby forest. When she came to investigate the wreckage, she saw a damaged spaceship and a girl who seemed to have been in it weakened by the impact. The girl gave Kanata two wristbands, alongside a request that Kanata join the Daiundoukai and become the Cosmo Beauty. Kanata obliged, but before she could ask for the girl's name, she mysteriously disappeared—leaving nothing,not even the crater caused by the crash. Since then, Kanata has devoted herself to training, aiming to fulfill her promise. After winning the qualifying round to represent the planet Earth, Kanata goes to the University Satellite—an institution where she and other candidates study to prepare themselves for the upcoming competition. There, she meets and befriends her fellow competitors, helping them get along with each other to perform better in the games. As Kanata grows closer to the coveted title, so do her chances of meeting the girl who changed her fate—and she discovers an underlying mystery behind the Daiundoukai along the way.