Toyama Kasumi, a first year high schooler with a shy personality, can no longer sing and express her emotions ever since she was made fun of her singing her favorite song. She is lonely and could not make friends even after entering high school. The only time she could show her emotions is through an exchange diary on her desk with a mysterious person named "Saaya" (サアヤ). Saaya seems to attend classes at a fixed time.

On one of her melancholic days, Kasumi saw star-shaped stickers pointing to a direction which she followed and end up at a pawnshop where she found a star-shaped guitar named "Random Star." Seeing the guitar, Kasumi wants to change herself and "shoot for her dreams".

Ichigaya Arisa, the girl she met at the pawnshop, offered Kasumi an alternative. Instead of buying the guitar, Kasumi will get the guitar in exchange of joining Arisa's game called "BanG_Dream!"

(Source: BanG Dream Wikia)