The protagonist awakens in an unfamiliar forest with amnesia, unable to recall his name or past. He discovers a translucent menu screen that informs him he has been reincarnated into a fantasy world and has earned the right to a new life. This system, which he later learns is embodied by a being named Melfina, a former angel now serving as a goddess of reincarnation, reveals his chosen profession as a Summoner with several powerful S-rank skills: Summoning, Appraisal Eye, and passive skills like Growth Rate Doubling and Experience Share. He also possesses F-rank Green Magic. Melfina explains that his past self chose to sacrifice his memories to gain more skill points, a decision that bewildens the protagonist.
Their immediate goal is to reach the city of Pars to register as an adventurer. During their journey, Melfina reveals more about his past life: he died in an accident caused by a god's mistake, leading to his special reincarnation. He apparently confessed his love to Melfina and convinced her to become his subordinate, a fact he struggles to accept due to his amnesia. Melfina, bored with her role, agreed to his proposal, seeing it as a form of "paid leave." The protagonist's primary objective becomes leveling up to be able to summon Melfina.
Upon arriving in Pars, he obtains a citizen's identification and heads to the Adventurer's Guild. He registers as "Kelvin," choosing the profession "Green Mage" to conceal his rare Summoner class, which could attract unwanted attention from powerful figures. He learns about the adventurer ranking system, starting at F-rank. He accepts a simple quest to defeat three Blue Slimes, which Melfina suggests as a good first target for his summoning skill.
Kelvin successfully contracts his first subordinate, a Blue Slime he names Kuroto, after weakening it and using his summoning magic. Kuroto, now a "Slime Gratonia," gains significant stat boosts due to Kelvin's S-rank summoning. Kelvin then uses his F-rank Green Magic, "Wind Blade," to defeat another Blue Slime, while Kuroto proves incredibly effective in combat. After defeating a third Blue Slime, Kelvin levels up to Level 2, gaining substantial stat increases and skill points, further highlighting his cheat-like abilities. He also learns about the skill point system, realizing his starting points and growth rate are exceptionally high. He prioritizes acquiring the F-rank "Concealment" skill to hide his abilities.
Over the next week, Kelvin focuses on completing quests, quickly advancing to E-rank adventurer status by defeating monsters like Orcs and Kobolds with Kuroto's assistance. His solo performance draws suspicion from the guild receptionist, Ange, who mistakes him for a seasoned court mage. During this time, Kelvin also acquires the F-rank "Absorption" skill for Kuroto.
Kelvin's attention turns to a D-rank quest involving a "Black Spirit Knight" at the "Ancient Castle of Evil Spirits." He encounters a charismatic but suspicious D-rank adventurer named Cashel, who, along with his companions Gimuru (a C-rank appraiser with the "Thief" class and "Bandit" title) and Radji (a C-rank fighter with the "Berserker" class and "Massacre" title), attempts to recruit Kelvin for the quest. Kelvin, having already used his Appraisal Eye to assess Cashel, Gimuru, and Radji and recognizing their dangerous titles and high stats, suspects their intentions. He proposes a solo race to defeat the Black Spirit Knight, intending to use the confrontation as an opportunity to deal with Cashel and his group.
Kelvin enters the dungeon, encountering undead monsters. He strategically waits for Cashel, Gimuru, and Radji to enter the dungeon, having prepared a trap with the D-rank Green Magic spell "Binding Mud." He uses his Concealment skill to hide the trap, which ensnares Radji. Gimuru flees, while Cashel attempts to advance. Kelvin then summons Kuroto, who has evolved into a powerful "Slime Gratonia," to block the exit and confront Cashel. The narrative ends with Kelvin preparing to face Cashel and his companions, having anticipated their ambush.
