Meet Louise, a budding magician. The students at the Tristein Academy call her "Zero Louise", due to her current record of zero successes with magic. In fact, her magic tends to go spectacularly wrong. Now, as an important test of her aptitude for magic, she must summon a familiar to be her servant. At this critical moment, she summons up all her magic and wishes for a familiar that is "devoted, beautiful and powerful", and gets... Hiraga Saito, an ordinary Japanese boy. It's difficult to say who is more surprised and dismayed, but the rules don't allow for second attempts. Louise is stuck with her strange familiar, and he with her.
Despite being from a famous magic family, Louise has never been able to successfully cast a single spell. On the day of her familiar summoning exam, she bungles the summoning and accidentally summons a human from Earth. The poor victim of her mistake is Saito Hiraga, a typical high school boy.
An abrupt departure from the normal life he once lead, Saito must deal with being Louise's familiar. Unhappy with the arrogance of nobles, Saito exposes Guiche's two-timing nature as an act of revenge. This of course, provokes Guiche into challenging him to a a duel. Despite being severely beaten up by Guiche's magic, Saito refuses to bow down. Impressed, Guiche tosses him a sword to even up the playing field. In a completely unexpected turn of events, the runes etched on Saito's hands begin to glow the moment he grasps the sword. Feeling a surge of energy, Saito manages to defeat Guiche. However, he soon collapses after the fight from fatigue.
With his recent victory over Guiche, Saito has become semi-popular among the commoners and several nobles. Kirche in particular has chosen him as her next boyfriend. In a devious plan, she attempts to seduce him, although this fails when Louise finds out. Annoyed by Saito's lack of defenses, Louise drags him out to town to pick up a sword after considering his hidden talents. Kirche, unwilling to let go so easily, matches Louise's gift with a more impressive sword. Of course, it simply causes more problems between Saito and Louise.
Siesta is sold to a royal family from out of town, presumably to be a mistress to the head noble. Disgusted by the lack of regard that nobles have for commoners, Saito tries his best to rescue her, but with no avail. As Saito attempts to take on the noble by fighting, Louise and the others arrive just in time to prevent Saito's death. In an act of kindness, Kirche gives up her family treasure to make up for Saito's actions and win Siesta back. Before leaving the manor, Saito catches a glimpse of the family treasure, and is shocked to find that it is in actuality a rather old porno magazine from Earth.
The yearly familiar exhibition is literally a day away and Louise is feeling rather depressed about it. Not only does Saito have nothing impressive to show off, but this year the Princess is watching as well, making it even more important. As night approaches, desperation sets in as Saito is unable to produce an act worthy of praise. A sudden mystery visitor turns out to be Princess Henrietta herself and her relationship with Louise is explained. The next day, Saito attempts a presentation only to be dragged off early by Louise. Walking away from the stage, they witness a robbery in progress by Fouquet and her earthen golem. Failing to stop her, Fouquet is able to run off with the fabled Staff of Destruction.
Worried that the princess may be blamed for Fouquet's theft, Louise volunteers to go out and catch her. With Kirche, Tabitha, and Saito tagging along, they travel to the last sighting of Fouquet with Longueville. There, they discover the hiding place of the Staff of Destruction only to have Fouquet's golem appear. Wishing to show her worth, Louise tries to take it head on with a spell, nearly being crushed in the process. Only by Saito's quick reflexes is she saved. Her confidence broken, Louise breaks into tears. Saito is moved by this and begins attacking the golem in her name. After discarding Kirche's sword and drawing Derflinger, the runes on his hands glow once again and his ability greatly increases. Louise tries to help by taking out the Staff of Destruction, which turns out to be a "M72 Light Anti-tank Weapon system(LAWs)" rocket launcher from Vietnam War. Immediately recognizing it, Saito grabs it and quickly uses it, blowing up the golem. Longueville claims the rocket launcher afterwards, revealing herself to be Fouquet. Yet she is unable to use it as it only had one shot, giving the gang a chance to capture her.
Having been concerned for the welfare of the commoners, Henrietta asks Louise and Saito to work undercover and figure out if what nobles are taking advantage of the lesser ranked commoners. Louise ends up wasting all of the money given to her for the mission on gambling in hopes of having enough to afford an expensive hotel. Refusing to ask for more money, Louise ends up working in a bar as a waitress. Unused to the brashness of a commoner's life, she is unable to earn any tips with her temper flaring all over the place. Her moment of salvation comes when a rather arrogant noble decides to enter the bar.
It's summer vacation and many students are traveling back to their families. Kirche goes along with Tabitha to her home on invitation, leaving Louise and Saito alone for once. However, mischief is still plentiful. Upset at Guiche's continued flirting with other girls, Montmorency creates a love potion and plots to have Guiche take it. Meanwhile, Saito obtains a large pot that was about to be thrown away and converts it into a makeshift hot tub, similar to the hot springs in Japan. While bathing in it at night, Siesta happens to pass by and much to Saito's embarrassment joins him. Seeing this, Louise becomes so frustrated that she interrupts Montmorency's plans by gulping the drink laced with the before mentioned love potion. As can be expected, when Saito returns to her room, Louise falls completely in love, causing many more problems.
The lovey-dovey antics of Louise under the love potion persist to the point of freaking Saito out. In desperation, Saito demands that Montmorency create an antidote. She refuses to do so until Saito learns of the illegality of creating love potions and threatens her with possible arrest. As the antidote requires the tears of a water elemental, Montmorency leads Saito and Louise to Lake Lagdorian. There, they find Kirche, Tabitha, and a whole new problem.
Having taken care of the love potion, a new problem arises again. This time, Princess Henrietta is to be wed to the Emperor of Germania for the sake of backing up the smaller country with a stronger power. In order to do so, a certain letter the princess sent to Prince Wales of Albion. If it is ever publicly revealed then the marriage would be impossible. At the moment, Albion is in a state of civil war, so Henrietta herself cannot see the prince. Therefore, Louise is sent with a message to Prince Wales requesting that the letter be retrieved. Guiche then enters, revealing that he had "accidentally" heard it all and would be willing to aid the princess. Much to both Louise and Saito's ire, Henrietta accepts Guiche's offer. On the day of departure, the escort arrives and is revealed to be Wardes, Louise's fiancé. A growing jealousy develops in Saito on the way to the port city of La Rochelle.
With the arrival of Wardes and Louise alone, they meet up with Prince Wales in a seemingly deserted church. After finishing up business, Wardes suggests to Louise that they should just get married right then. Confused and repulsed by the idea, Louise tries to refuse his offer, but one of Wardes' allies comes out with a mind-control ring, forcing her into agreeing. With the Prince as the minister, the marriage proceeds and is nearly completed until Saito bursts into the church. Snapping Louise out of her trance, Saito rushes to rescue her. In response, Wardes kills Prince Wales, revealing that he is in fact involved with the rebels fighting the government Albion called Reconquista.
The gang has located a rare dragon, which Siesta's family has kept a map. It turns out to be a Mitsubishi Zero-sen, a Japanese aircraft use in World War II. They bring it back to the school. Colbert said someone earlier used another aircraft to fly during the solar eclipse and disappeared. Saito concluded that the person returned home and this is his chance for him to return home also.
The enemy is starting attack villages and another solar eclipse is about to happen. The empire has formed an army with Louise and Guiche to avert the attack. Saito finds out and uses the plane to come to their aid, but he has to give up his chance to return home. He was able to use the machine gun to eliminate most of the enemy, but ran out of ammo and couldn't take down Wardes. Louise unlocks her magic, destroying the entire enemy force. After the battle, the solar eclipse is over and the plane is nearly destroyed. Saito ends up staying.