Humanity is finally confronted with the threat of the Custodians—the ancient, sentient species held responsible for cursing humanity to speak different languages thousands of years ago. The Symphogear wielders—Hibiki Tachibana, Tsubasa Kazanari, Chris Yukine, Maria Cadenzavna Eve, Kirika Akatsuki, and Shirabe Tsukuyomi—are sent to the Antarctic in order to retrieve an ancient relic. After securing it and rescuing the scientific staff present there from a Coffin, the automated defense mechanism protecting it, the relic is given to American researchers due to international agreements. The criminal organization Noble Red, a remnant of the previously fought Bavarian Illuminati, starts targeting the relic. Will the Symphogear wielders and their supporting organization S.O.N.G. be able to foil the plans of the organizations conspiring against them?

Kousei Arima is a child prodigy known as the Human Metronome for playing the piano with precision and perfection. Guided by a strict mother and rigorous training, Kousei dominates every competition he enters, earning the admiration of his musical peers and praise from audiences. When his mother suddenly passes away, the subsequent trauma makes him unable to hear the sound of a piano, and he never takes the stage thereafter. Nowadays, Kousei lives a quiet and unassuming life as a junior high school student alongside his friends Tsubaki Sawabe and Ryouta Watari. While struggling to get over his mother's death, he continues to cling to music. His monochrome life turns upside down the day he encounters the eccentric violinist Kaori Miyazono, who thrusts him back into the spotlight as her accompanist. Through a little lie, these two young musicians grow closer together as Kaori tries to fill Kousei's world with color.

The rivalry between three piano geniuses in Towa Hall starts with the prodigy Kousei Arima. While his emotive first performance enamors the free-spirited Emi Igawa, Kousei eventually evolves his technique to become exact and precise, finding himself a new follower in the eccentric Takeshi Aiza. Unbeknownst to outsiders, the two other players begin to target Kousei with their respective performances. Emi attempts to convert Kousei back to his old self with her vibrant playing, while Aiza turns his admiration for Kousei into motivation to beat him at his own game.

Shin Koihime†Musou characters sing theme songs on stage at a live event.