After the unexpected death of her father, 11-year-old Momo Miyaura leaves Tokyo with her mother and moves to an old remote island in Seto Inland Sea. The only memento she has from her father is an unfinished letter with only two words inside: "Dear Momo"—along with her heart's unrest from it. In the new and unfamiliar small town, Momo reluctantly tries to adjust to the outmoded wooden buildings, silent crop fields, and mysterious isolated shrines. One day, while exploring the attic of her new home, she finds a worn out picture book about youkai. Following this discovery, strange things begin to happen around town, and Momo is greeted by the arrival of three troublesome youkai. Momo e no Tegami tells the story of a young girl as she struggles to adapt to her bizarre new life and ultimately come to terms with her father's mysterious letter.

When Kasumi Toyama was out camping as a child, she heard the rhythm of the "Star Beat" while admiring the starry night sky. This sparkling, heart-pounding sound left a lasting impression on her, and since then, she has constantly been seeking to rediscover that exciting thrill. Now, as a newly enrolled student of Hanasakigawa Girls' High School, Kasumi is more determined than ever to find that long-lost sensation again. She attempts a myriad of school club activities, and even considers taking up a part-time job, but none of these feel suitable for her. That is, until she chances upon a star-shaped guitar in the storage room of an old pawn shop. It leads her to a live house, where she is captivated by watching a live performance for the first time. Spurred from the adrenaline, she instantly decides to form a band without even really knowing why. However, such a venture requires her to confront various hurdles with new friends she makes along the way, all in her pursuit to encounter that twinkling, heart-throbbing ardor once more.

Kasumi Toyama and the members of Poppin'Party—their high school band—are now second-year students, but with the new school year comes their latest set of challenges: student council duties, cram school, and part-time jobs, among other things. Furthermore, they are now separated into different classes, and as a result, their free time together is shortened. Looking back, their joyful times together seem like nothing more than an unachievable fantasy when their members are too occupied to even gather in Arisa Ichigaya's basement for practice sessions. As the band ponders over the situation, an unfamiliar girl suddenly emerges, unknowingly pulling them into the spotlight once more. In this rising era of girl bands, Afterglow, Pastel*Palettes, Roselia, and Hello, Happy World! begin to make their appearance as well. Each band, comprised of musicians with their unique personalities, illuminates the stage with its performances. As Poppin'Party returns to the music scene and encounters new friends, the sparkling, heart-pounding music starts to play once again!

As the all-girl band Poppin’Party continues to rise in fame, they find a poster for an event at the legendary Nippon Budoukan. Now determined to participate in the "BanG Dream," Kasumi Toyama and the other members of Poppin’Party aim to reach new heights and prove their worth by performing on that renowned stage. Meanwhile, rival band RAISE A SUILEN dreams of changing the world with "the strongest music," but finds itself in dire straits without a guitarist. Though dozens of potential candidates audition for the role, band leader CHU² believes them to be amateurs and unsuitable for the band. However, there may be one guitarist that lives up to CHU²’s high standards. BanG Dream! 3rd Season features multiple all-girl bands, including Poppin’Party, Afterglow, Pastel*Palettes, Roselia, Hello Happy World!, and now RAISE A SUILEN, as they work to astound fans throughout Japan with their spectacular performances. As the BanG Dream competition draws near, the band members are prepared to captivate audiences with their heartfelt music!