The protagonist, a seemingly ordinary high school student named Shido Toyoshima, finds himself abruptly summoned to another world along with his entire class. Their purpose, as explained by a beautiful princess, is to defeat the Demon Lord. While the method of returning home is uncertain, the contract magic binding them to this world is said to be broken upon the Demon Lord's defeat. Upon arrival, the summoned individuals are granted powers. Shido's ability is revealed to be Summoning Magic, a rare and potentially powerful skill, though his initial manifestation is a hamburger, leading to disappointment and his dismissal as a useless hero.
Despite this setback, Shido's summoning magic proves to be versatile, allowing him to summon items from Earth, such as hamburgers and later, convenience store onigiri. He discovers his magic is wish-based and its power scales with his magic consumption. Recognizing the kingdom's suspicious intentions and his own precarious position, Shido is given severance pay and expelled. His primary motivation becomes returning to Japan to support his family. He begins to gather resources, including food and a world map, and trains his summoning magic by repeatedly summoning inexpensive items to increase his magic capacity.
Shido learns that his summoning magic can be enhanced by leveling up, which involves absorbing "magic essence" from defeated monsters. However, he distrusts the kingdom and decides to leave for the Chrome Kingdom, a rival nation, to pursue his goals without interference. He observes the harsh reality of slavery in this world, where slaves are used as shields and experience generators for their masters. He learns about the "slave crest" that transfers magic essence to the master and the "hero education program" that utilizes slaves for knight and hero training.
Upon arriving in the Chrome Kingdom, Shido registers as an adventurer and plans to purchase a slave to aid in his level-up efforts. He travels to the city of Bertoria and, after some initial hesitation about the morality of buying slaves, decides to proceed. He attempts to summon a handgun, a powerful weapon from his world, and successfully does so, along with ammunition. He then ventures out to test his new weapon and his own combat abilities, encountering and defeating three goblins. This experience allows him to level up to five and further hone his skills with the handgun.
Shido then seeks out potential slaves, initially considering purchasing from a slave merchant. He learns about the pricing and conditions of slave contracts, particularly the rarity and cost of human slaves. He also learns about the possibility of temporary slave contracts for leveling purposes. However, he ultimately decides to seek out individuals in the slums, believing he can find willing participants there. He encounters three young women, identified as elves, who are in a desperate situation. He offers them food and shelter in exchange for their participation in his leveling efforts, intending to form a slave contract with them. He uses his summoning magic to provide them with familiar and delicious Earth food, effectively "hooking" them on his creations. After a discussion and a "strategy meeting," the three elves agree to a slave contract, with Shido providing for their needs in exchange for their help in leveling him up. The contract is finalized at the slave merchant, establishing a master-servant relationship with specific terms regarding rest, food, and the absence of sexual slavery. Shido then takes the three elves, now his slaves, to test their combat abilities.
