In a world of adventurers and magic lies Naral Island. Every generation, a Demon Lord rises to plague the land, and every generation, a Hero is born to subdue him. For countless centuries, the cycle has repeated with no end in sight. The latest Hero, Juulia Yusha Charldetto, has almost completed her valiant campaign alongside her party members: responsible priest Seiran Seira Élénoir, enigmatic mage Meiza Mei Endust, and hyper-energetic warrior Fai Fai. In the final battle against the Demon Lord, Yusha's party attempt a risky spell to cast their enemy into the drifts of time. But the incantation goes awry, sending Yusha and her friends back to a time before the Demon Lord, before Yusha becomes the Hero, and before the party had even graduated as adventurers. With their memories of the future erased, the four girls restart their ambitions to become the Hero's Party, aspiring to defeat the Demon Lord. However, in a sudden twist of fate, the Demon Lord was also sent back in time with her memories intact. Reduced to the form of a little girl, the Demon Lord takes the name Mao and infiltrates the adventurers' school as a teacher, planning to stop Yusha before she becomes a hero. Thus begins the story of Yusha and her friends, in their quest to defeat the Demon Lord, not knowing that the one they seek is right by their side.

Within Cardia Beckford's hazy memories, she can recall her father Isaac and the home where she lives alone, feared as a monster by the townsfolk—for in her body, she carries a deadly substance. Embedded in her chest by her father, the eternally beating heart—also known as Horologium—has the capability to produce infinite power. However, it also makes her skin destroy anything it touches. Many in London seek the Horologium, including the terrorist organization Twilight, with whom Isaac is rumored to have close ties. To obtain the Horologium's power, the British military forces Cardia to leave her home as their prisoner. But on the road, she is whisked away by the gentleman thief Arsène Lupin, who says he will steal her heart. Joining Lupin and his companions, Cardia begins a journey to discover the truth behind Isaac's connection with Twilight, her missing memories, and the Horologium within her chest.

Everyone has a dream they strive to achieve, and so does the musically talented Kanon Shibuya. However, due to her stage fright, Kanon fails to make it into Yuigaoka Girls' High School's music program and instead ends up in the general curriculum. Even though Kanon makes the conscious decision to quit music altogether, her classmate Tang Keke rekindles Kanon's passion for music with her own: a passion for school idols. Although Keke and Kanon try to form a school idol group, their effort meets strong objections from Ren Hazuki, the daughter of the school's director. Even so, the girls soon visit the director, who allows them to set up their school idol club under one condition—they must win first place at the Yoyogi School Idol Festival. The resilience of these aspiring idols is put to the test as they face the challenges that come their way. Throughout their journey, the girls make new friends and meet new members that join them to become the dazzling idols others can only imagine.

Tatsuko of class 5-1 is really excited about the upcoming school sports festival, her proposal for a new Eel Scooping event is less than enthusiastically received though, especially by Suzuka. Illya mentions the main event dance to Miyu, and as teacher Taiga Fujimura points out, it entails a huge reward, but will require quite a bit of practice. And so the close friends: sarcastic Nanaki, level-headed Suzuka, excitable Tatsuko, energetic Illya, shy Mimi, and reserved but athletic Miyu are off to win the dance event for the honour of their class and for Fujimura, who seems to have some saucy bet going on that she most definitely needs to win, or so the girls misunderstand. (Source: AniDB)