After receiving some injuries following a fight with some punks, orphan Fuuka Reventon is helped out by Einhart, who invites her to come to the Nakajima Gym afterwards. Noticing Fuuka's potential as a martial artist, Einhart offers her a part-time job at the gym. Despite having a hatred of martial arts after losing to her former friend, Rinne Berlinetta, Fuuka accepts the job and starts off as a sparring partner for Vivio and her friends, becoming surprised by how strong they are. Hearing that Vivio is the one who broke Rinne's winning streak, Fuuka faces her in another sparring match, deciding to stop running away and aim to become stronger.

Fuuka gets used to working at the Nakajima Gym while also receiving martial arts training from Einhart and her friends. The girls soon begin training using Power Mode, which lets them transform into adults, discovering that Fuuka possesses a high level of magic. After receiving invites to watch Rinne's semi-final match, Nove shows Fuuka footage of Rinne's previous battles, with Fuuka revealing her feud with Rinne spawned from her insulting the ability of one of her opponents.

The girls gather at the arena to watch Rinne's semi-final match against Carrie Tercel, which ends as soon as it begins due to Rinne's unrelenting battle style. Following her match, as Rinne states her determination to beat Einhart, Vivio, and Miura in what she claims will be her last Under 15 tournament, she becomes surprised when Fuuka announces her intention to join the tournament too. Afterwards, Fuuka and Vivio question Rinne over why she is pushing herself to perform martial arts, to which she responds violently, stating her determination to prove her strength at the tournament.

Einhart's friend Yumina Englave informs Fuuka about the circumstances that led to Rinne's current demeanor. Four years ago, Rinne was adopted into the Berlinetta family, receiving the device Scuderia made by her adoptive grandfather. However, as Rinne became popular in her new school due to her athletic potential, she became the target of bullying. Then one day, the bullies became more violent towards Rinne, mistreating her device and preventing her from answering an emergency call as her grandfather ended up passing away. Blaming herself for her grandfather's death, Rinne took brutal revenge on her bullies, becoming determined to become stronger.

Following the incident, which was settled outside of court, Rinne ran away from home but was kidnapped by thugs, led by the brother of one of the bullies she had beaten up. Just then, she was rescued by a woman named Jill Stolas, who became her martial arts coach, helping her to become stronger. Having heard everything from Yumina, Fuuka, satisfied that Rinne's circumstances are her own but still wanting to find a way to help her, decides that she wants to beat her and set her straight. A few days later, Fuuka finally receives her device, Huracan, and begins more advanced sparring against various fighters with tournament experience.

After Fuuka manages to place first in her qualifiers, the tournament placements are announced, in which Miura is scheduled to fight against Rinne during the first round. On the night before the tournament, Miura spends the night at Fuuka's place, stating her desire to once again fight Vivio before she graduates from the Under 15 category. As the tournament begins, both Vivio and Fuuka win their first round matches with swift knockouts. During the next match, Miura gets an early advantage against Rinne, knocking her down twice with an array of strikes and counter blows. However, as Miura brings out her Sword Draw technique in the hopes of getting a knockout blow, Vivio realises that following her defeat in the Summer Cup, Rinne has been strengthening her defense. Managing to withstand Miura's attacks, Rinne delivers a powerful blow that shatters Miura's ribs, forcing Nove to throw in the towel.

After Einhart quickly wins her first round match, the girls check up on Miura, who is secretly upset about her failure. As Vivio decides to spend the night training, she and the others recall how she and Nove first started training and everything that had happened since. On the next day of the tournament, Vivio faces off against Rinne, delivering several quick jabs. Rinne soon starts delivering a powerful counterattack, but Vivio manages to break free, stating her determination to win and perhaps become friends with her.

Vivio continues to take control of the match through the second round, with Rinne struggling to keep up with her pace. In the third round, however, Rinne takes a direct hit to the face so she can deliver a brutal counter, dealing heavy damage to Vivio. Despite the damage taken, Vivio manages to stand up again to resume the fight. Changing up her stance, Vivio defeats Rinne with a knockout barrage, albeit sustaining enough damage that she herself will need to drop out of the finals. As both Rinne and Jill are distraught over their loss, Fuuka prepares for her next match.

Both Fuuka and Einhart manage to win their matches and reach the finals, where they are scheduled to fight against each other. Meanwhile, Rinne has been holed up inside her room following her defeat against Vivio. As Nove encounters Jill at the hospital, they hear from Victoria that Rinne plans to quit martial arts. Hearing Fuuka's resolve to still fight Rinne, Einhart offers Rinne the opportunity to have a title match against her if she can defeat Fuuka. Accepting Einhart's proposal, Rinne arrives at Lutecia's training ground for a no-holds-barred match against Fuuka.

With the two girls constantly exchanging blows, Fuuka gains the upper hand by using techniques she had learned from sparring with her friends. Despite Jill wanting to throw in the towel, Rinne remains determined to fight back. While Rinne remains focused on her obsession with gaining strength, Fuuka attempts to reason with her, recalling when the two of them spent time together at the orphanage. Feeling that her biggest enemy is herself, Rinne states that if she wins, Fuuka must stop meddling in her affairs.

As the match continues, Jill recalls when she first started training Rinne, regretting not being able to save her from her self-inflicted torture. Despite being overwhelmed by Fuuka's strength and her own wavering heart, Rinne recalls all of her intense training with Jill and counters with a incredibly powerful uppercut that surpasses the field's safety setting. With both fighters no longer holding any bitterness towards each others, the girls decide to settle things with an all-offensive fist fight. As Rinne finally comes to accept that she is allowed to be happy, Fuuka wins the match by knockout, happy that the light has returned to Rinne's eyes.

Following the match, Rinne talks with Jill, stating that she intends to continue competing while training under her. Later, Fuuka and Rinne spend some time together, receiving messages from those who watched their match and rekindling their friendship. Afterwards, as Fuuka and Einhart both prepare for their match together, Jill apologises to Nove, coming to understand more about the way she trains her students. On the day of the Winter Cup final, Rinne and Jill come to support Fuuka as she goes up against Einhart.