Zalmen is a reincarnated adventurer who has spent years stuck at level 1 with no prospects, leading him to decide on two goals: retire from adventuring entirely and lose his virginity in one final splurge. He takes his entire life savings to an exclusive high-class brothel, where he meets a stunningly beautiful woman. During his first sexual experience with her, at the moment of climax, a strange voice rings in his ears: "Fertilization confirmed. Level up!" This bewildering announcement makes him realize that his leveling system is tied to impregnation through sex. The more he ejaculates inside a partner, the stronger he becomes. This discovery turns his life around completely. What was once a permanent bottom-tier adventurer now has a path to power, though the method is anything but conventional. The story takes place in a fantasy world full of dungeons, monsters, magic, and adventurer guilds, where level and skill determine survival and success. Zalmen returns to the adventurer guild and begins to test his newfound abilities, quickly rising from his former status. Along the way, he reunites with former party members, including the mage Marie, who becomes a key companion and romantic interest. When Marie and other former comrades become trapped in the hidden dungeon beneath the ruins of Mara, Zalmen ventures in to rescue them. The dungeon proves extremely dangerous, and during the ordeal he becomes separated from Marie and exhausted of magic, only to be attacked by a mysterious fog-like monster that sexually assaults him. Once again, at the critical moment, the level-up announcement triggers, saving his life. After barely escaping the dungeon, Zalmen learns that the authorities want a return expedition to retrieve the ancient knowledge known as the Wisdom of Mara. Realizing he needs far greater strength to survive a second trip, he begins working with Shirley, an older woman who acts as a sister figure, to implement a systematic "Happiness Level-Up Plan." This involves regular sexual encounters with women at the brothel who have their own reasons for wishing to become pregnant, allowing him to steadily accumulate levels. With his power growing, Zalmen receives a sword passed down from his senior Hagen and gains enough confidence to challenge the Mara dungeon again. The second expedition leads to a prolonged battle against a black monster that gradually drains the party's stamina. One by one his allies fall, and in a desperate final attack, Zalmen unleashes all his magic in a single blast of light that seems to destroy the monster. However, the effort completely drains his life force, and he finds himself before the god who originally reincarnated him, facing the possibility of being sent to yet another world. The series follows Zalmen's journey from a hopeless failure to a capable adventurer, driven by a bizarre and highly explicit level-up mechanic tied entirely to sexual reproduction, all set against a standard fantasy backdrop of dungeons, guilds, and monster battles.