Mizore is a quiet, introverted student in her third and final year of high school, who is an oboist in the school’s concert band. Her only friend, who occupies most of her thoughts, is Nozomi, one of the band’s flutists, who is much more outgoing and popular. Together, the two rehearse a duet from the musical piece Liz and the Blue Bird, which is based on an eponymous, fictional German fairy tale that Nozomi loved as a child. The story is about a young woman named Liz (represented by the oboe in the musical piece) and an unnamed blue bird turned human (represented by the flute) who become best friends and live together, until the two are forced to part ways. Nozomi and Mizore realize that the story applies closely to their own relationship, which worries Mizore; she sees herself as Liz and Nozomi as the fleeting blue bird.
Although Nozomi spends time with other friends from the band, Mizore keeps herself isolated from everyone except Nozomi, and refuses her other bandmates’ offers to spend time together. She also tries to express her love to Nozomi, but is never able to. Niiyama, a woodwind instructor, advises Mizore to apply for music school after graduation so she could become professional; she is not particularly interested at first, but changes her mind after Nozomi says that she might apply as well. However, their bandmates are worried, realizing that Mizore’s only motivation for going to music school is to be together with Nozomi.
As the concert approaches, Mizore and Nozomi grow apart. Mizore feels insecure about Nozomi because of their history together: during middle school, Nozomi proposed and convinced Mizore to join the band, only to later leave it during their first year of high school due to a schism (as depicted in Sound! Euphonium). As a result, Mizore is afraid that Nozomi could leave her at any moment. Meanwhile, Nozomi shows discomfort at Mizore progressively opening herself to others and being tutored by Niiyama, envious of her greater potential. Furthermore, the two have trouble perfecting their duet, both because of their increasingly complicated relationship and because of their difficulty connecting with the characters from Liz and the Blue Bird; Mizore, in particular, does not understand why Liz would ever let the blue bird go free instead of keeping it with her forever.
Eventually, the two come to a greater understanding of their relationship, thanks to the assistance of Niiyama and other members of the band. They ultimately come to realize that, while they associated Mizore to Liz and Nozomi to the blue bird, Mizore was actually closer to the blue bird—having to let go of her unconditional attachment so she can live her own life—while Nozomi was closer to Liz, who let the bird go free so that she would not weigh it down. At the next rehearsal, they perform the piece perfectly. Mizore’s performance moves her bandmates and leaves Nozomi in tears. They confront each other afterwards, Nozomi having realized that Mizore was underperforming so the two would be on the same level. Nozomi also reveals that she did not really want to go to music school, and had only said so out of jealousy, even though she knew she did not have the skill necessary to be accepted. Mizore, upset that Nozomi seems to be abandoning her once again, confesses the extent of her feelings, calling the other her “everything”. However, Nozomi only laughs, knowing that accepting Mizore’s feelings would only continue to confine her.
Some time later, Mizore and Nozomi are seemingly still distant from each other, with Nozomi prioritizing her studies over the band. After they meet in the school’s library, Nozomi offers to eat together outside. On the way, she claims that she will back up Mizore perfectly in their duet, only asking for “a little time” and implying that she intends to overcome her jealousy and support Mizore in her life and decisions, while Mizore answers that she will keep on playing the oboe, hinting at her acceptance to finally follow her own path instead of Nozomi’s.