A shy high school girl named Cyan Hijirikawa loves music and dreams of playing in a band, but lacks the courage to approach her school's music club. One night while playing a rhythm game on her phone, she is suddenly pulled into the screen and transported to Midi City, a vibrant, neon-lit metropolis where music holds supreme power and the inhabitants are anthropomorphic animal-like beings called Myumons. Upon arrival, she is transformed into a white cat girl and finds a magical guitar called the Strawberry Heart, which houses the spirit of a musician named Grateful King.

After using the guitar's power to drive off a music-devouring Dark Monster, Cyan catches the attention of Maple Arisugawa, the eccentric egg-shaped president of the record label Branded Rocking Records. He recruits her to join his agency's up-and-coming band, Plasmagica. The band already consists of three other members: Chuchu, a prideful and ambitious rabbit girl who acts as the group's de facto leader; Retoree, a shy and introverted dog girl who is a tech enthusiast; and Moa, a cheerful and mysterious sheep girl from outer space. Together, they aim to rise to the top of the city's competitive music scene.

As Plasmagica performs at battles of the bands and builds their reputation, they encounter numerous other acts, including the cool male rock band ShinganCrimsonz, the traditional Japanese-inspired Tsurezurenaru Ayatsuri Mugenan, and the idol-like rival band Criticrista. While navigating the challenges of friendship, rivalry, and creative expression, the band also becomes entangled in a larger conflict. The Dark Monsters threatening the city are revealed to be the work of Dagger Morse, the sinister CEO of a rival label who seeks to corrupt musicians' hearts and plunge Midi City into darkness.

Throughout the season, each member of Plasmagica confronts personal struggles. Retoree fears losing Cyan, who comes from another world and may one day have to return home. Chuchu grapples with jealousy over Cyan's natural talent and secretly plans to use the band as a stepping stone for a solo career, a vulnerability that Dagger exploits. Moa struggles with whether to stay with her new friends or return to her alien family. Cyan must balance her desire to perform with her secret mission to defeat the Dark Monsters, often having to abandon concerts to fight.

The narrative builds toward the Grateful Rock Festival, a major concert where Dagger enacts his plan to use a dark song to take over the city. He corrupts Criticrista's leader, Rosia, transforming her into a powerful Dark Monster. In the climax, Plasmagica and the other bands unite to free Rosia and confront Dagger. Cyan, empowered by the bonds she has formed, uses the Strawberry Heart to defeat him and restore peace to Midi City. Having grown more confident through her experiences, Cyan returns to her human world, finally ready to open the door to her school's music club and join the band there. However, the story continues in a second season, Show By Rock!!#, where a new threat forces Cyan to return to Midi City for further adventures.