On the magical world of Midchilda, Vivio Takamachi is a 10-year-old girl who lives with her adoptive parents, Nanoha Takamachi and Fate T. Harlown, and goes to school with her friends Corona Timil and Rio Wesley. To celebrate Vivio entering her fourth year of elementary school, Nanoha decides to give her her own rabbit-shaped Intelligent Device, which she decides to name "Sacred Heart" or "Chris" for short. Using her new device, Vivio transforms into her mage form, taking the adult Sankt Kaiser form she previously used during an incident several years ago, which comes as a surprise to Fate. As Vivio does some late night training with Nanoha, her Strike Arts instructor, Nove Nakajima, hears about a girl going under the name of the Sankt Kaiser Heidi E. S. Ingvalt who has been attacking various people. The next day, after Nove accompanies Vivio to visit her comatose friend, Ixpellia, she is challenged by the self-proclaimed Ingvalt.

After questioning her about Vivio and Ixpellia, Ingvalt battles against Nove, determined to prove her own strength. Despite winning the match, Ingvalt, who, like Vivio, is a child using an adult form to battle, passes out from exhaustion. Recovered by Nove's sister Subaru and her partner Teana Lanster, Ingvalt, who is revealed to be a middle school student named Einhart Stratos, is examined by doctors, who reveal she possesses memories of the Sankt Kaiser Claus G.S. Ingvalt and has inherited his Belkan fighting style. As Nove learns more about Einhart and her ancestor's past, Vivio and her friends brush on the history of Sankt Kaiser in preparation for a meeting with Einhart.

Nove invites Einhart to join Vivio in her Strike Arts training, having them participate in hand-to-hand sparring. Managing to take on Vivio's attacks and beat her with a single strike, Einhart feels disappointed that Vivio wasn't being serious, so Nove arranges a proper match for them the following week. As Vivio becomes downhearted that she couldn't live up to Einhart's expectations, she decides to train hard over the next week in order to become stronger. On the day of the match, Vivio brings out her full strength against Einhart, but once again loses to Einhart's amazing strength, though not before leaving a delayed counterattack. Despite still feeling she isn't the girl her ancestor wanted to meet, Einhart finds herself drawn to Vivio and decides to become friends with her.

Following school exams, Einhart is invited to join Vivio and the others to a special camping trip to Lutecia Alpine's residence in Carnaaji. The girls start off the trip with a swim in the river, where Einhart learns how to control her strength whilst in water. Afterwards, they have lunch, where Vivio tells Einhart about how Nove became her instructor.

Einhart shares with Vivio about the memories passed down to her by Klaus, including all the happy memories spent with Vivio's ancestor, Sankt Kaiser Olivie Segbrecht, whose death drove Klaus to try and obtain true strength. Later, the girls get to watch Nanoha, Fate, and the other top mages in intense mock battles, which encourages Vivio and Einhart to resume training, while Corona receives her own intelligent device from Lutecia. Afterwards, everyone gets to enjoy the residence's hot springs, where Sein, one of Nove's sisters, attempts to get up to mischief but is stopped in her tracks when Rio's adult mage form activates in self-defense. Once the matter is settle with, Einhart becomes excited about a joint mock battle with the top mages taking place the next day.

The mock battle splits the mages into two teams; the Blue Team, featuring Vivio, Nanoha, Subaru, Rio, Lutecia, and Erio Mondial, and the Red Team, consisting of Fate, Einhart, Corona, Teana, Nove, and Caro Ru Lushe. As everyone focuses on their chosen opponent, Corona uses her new device to summon a Golem to fight against Rio, though still struggles against her fighting style. Believing the Blue Team's strategy revolves around Nanoha, Teana sends in Einhart who, despite managing to break out of her bind with a move she learned from Nove, is ultimately overwhelmed by Nanoha's power. The Blue Team soon carry out their plan, facing against the Red team's members two-on-one, while Teana prepares a counterattack.

With two fighters eliminated on each side, Nanoha and Teana launch Starlight Breakers at each other, the aftermath leaving just Vivio and Einhart remaining in the game, ultimately ending in a double knockout. After a few more mock battles featuring different matchups, Lutecia tells Einhart about the Inter-Middle Championship, a tournament for young mages which everyone is interested in entering. Wanting to fight more with Vivio, Einhart decides to enter the tournament as well. With Einhart needing a device in order to qualify, Lutecia gets into contact with Hayate Yagami, who offers to make a device that suits her fighting style. In the meantime, Nove begins training Vivio and the others for the tournament, with many strong fighters looking to enter.

Vivio has a sparring match against one of the other tournament combatants, Chantez Apinion, using Chris' Contact Mode to block each of her attacks but becoming caught off guard by Chantez's special technique. Meanwhile, Hayate and her team finish completion on Einhart's new device, which she decides to call Asteon or Teo for short. Afterwards, each of the girls begin their individual training for the tournament, with Vivio studying under Nove, while Chris approaches Nanoha on how better to protect Vivio. As the day of the tournament qualifiers soon arrive, Vivio is introduced to Vita and Zafira's understudy, Miura Rinaldi, with both of them clearing their qualification rounds.

The star players soon have their qualifying matches, with Els Tasmin facing off against former champion Harry Tribeca. Els hits Harry with her chains, but Harry manages to break her arm free and deal heavy damage in return. Els attempts a counterattack, but Harry uses her own chains against her to win the match. After all of Team Nakajima make it through the qualifying rounds, Miura faces off against Mikaya Chevelle, the one who trained Einhart for the tournament. Despite taking a lot of damage from Mikaya's sword attacks, Miura brings out the full might of her Star Saber device to defeat Mikaya with a powerful kick.

With the third round approaching, Vivio is scheduled to fight against Miura while Einhart and Corona must face against each other. Before that, Chantez faces off against Victoria Dahlgrun, managing to land some attacks with her clone magic. However, Victoria remains resistant and overcomes Chantez's clones, winning the match. Meanwhile, Corona prepares to face off against Einhart, preparing a risky technique that Nove suggests she only use as a last resort. As the match begins, Corona uses a technique against Einhart in order to create an opportunity to make her golem.

After a fierce battle, Einhart defeats Corona.

Einhart feels bad after defeating Corona and her previous opponents, but Nove teaches her the meaning of winning and losing in a competition. Elsewhere, Vivio and friends prepare for their own upcoming matches.