The Familiar of Zero

Louise, a member of a prestigious magical family, struggles with casting spells and fails her familiar summoning exam by summoning an unexpected human from Earth named Saito Hiraga, an ordinary high school student.

Saito, forced to become Louise's familiar, takes revenge on the arrogant noble Guiche by exposing his infidelity, leading to a duel. Though badly beaten, Saito refuses to surrender, prompting Guiche to offer him a sword. When Saito touches the sword, his hand runes activate, granting him the strength to defeat Guiche before collapsing from exhaustion.

Saito gains some popularity among commoners and nobles after defeating Guiche, drawing the attention of Kirche, who tries to win him over but fails when Louise discovers her advances. Frustrated by Saito's lack of combat skills, Louise takes him to town to obtain a weapon, only for Kirche to counter with a more impressive gift, further complicating their relationship.

Siesta is taken by a visiting noble family, likely to serve as a mistress, prompting Saito to attempt a rescue. His efforts lead to a confrontation with the noble, but he is saved by Louise and the others. Kirche offers her family's treasure to help Saito, only for him to later discover the treasure is an old Earth-era adult magazine.

With the annual familiar exhibition fast approaching, Louise is anxious and dejected, especially since Saito has failed to prepare a suitable performance and the Princess is in attendance. As time runs out, Saito is visited by Princess Henrietta, who clarifies her connection to Louise. On the day of the exhibition, Saito attempts to perform but is interrupted by Louise and they soon encounter Fouquet committing a robbery with an earthen golem, stealing the Staff of Destruction.

Louise takes the others to the last known location of Fouquet and Longueville, hoping to prevent the princess from being implicated in a theft. They discover the Staff of Destruction, but are attacked by Fouquet's golem. After Louise is nearly injured and Saito intervenes, he uses a recovered weapon to destroy the golem, saving the day.

Henrietta tasks Louise and Saito with investigating whether nobles are exploiting lower-ranking commoners. Louise squanders her mission funds on gambling and ends up working as a waitress in a bar, struggling with the challenges of commoner life and her own short temper. Her situation improves when an arrogant noble enters the establishment.

During summer vacation, Kirche and Tabitha leave for Tabitha's home, leaving Louise and Saito alone. Montmorency, annoyed by Guiche's flirtations, plots to give him a love potion, while Saito repurposes a discarded pot into a hot tub. Louise accidentally consumes the potion meant for Guiche, leading to unexpected complications.

Louise's behavior under the influence of a love potion becomes increasingly unsettling for Saito, prompting him to ask Montmorency to create an antidote. When she refuses, Saito warns her about the legal consequences of making love potions, leading her to agree on the condition that they retrieve an ingredient from Lake Lagdorian. Their journey there brings them face to face with Kirche, Tabitha, and an unexpected new challenge.

After resolving the issue of a love potion, a new challenge emerges when Princess Henrietta is betrothed to the Emperor of Germania to secure political support for her country. A complicating factor is a letter she sent to Prince Wales of Albion, which could prevent the marriage if discovered, but she cannot visit him herself due to a civil war. Louise is tasked with delivering a message to retrieve the letter, and Guiche offers his help despite Louise and Saito's disapproval. On the journey to La Rochelle, Louise's fiancé, Wardes, accompanies them, sparking jealousy in Saito.

Wardes and Louise arrive at a seemingly abandoned church where they meet Prince Wales. After some discussion, Wardes proposes marriage, but Louise is horrified and tries to refuse. One of Wardes' allies uses a mind-control device to force her compliance, and the wedding begins with Prince Wales officiating. Saito interrupts, breaking Louise's trance and attempting to rescue her, prompting Wardes to kill the prince and reveal his allegiance to the rebel group Reconquista.

The group discovers a rare dragon, which is later identified as a Mitsubishi Zero-sen, a Japanese aircraft from World War II, based on a map from Siesta's family. They bring it back to the school, where Colbert mentions a previous attempt to use an aircraft during a solar eclipse that ended in disappearance. Saito theorizes that the person may have returned to their original time and sees this as an opportunity to do the same.

Cromwell initiates an invasion during a solar eclipse, prompting Louise to prepare for battle as Saito approaches the possibility of returning to his home world.