In a fantasy world where a person's job is granted by the gods at birth and can never be changed, Ein is born with the Appraiser class, considered the weakest and most worthless of all. His only ability allows him to assess the value of items and the rank of monsters, making him a despised pack mule for his adventuring party. While exploring a dungeon, his companions Zoid and Jolene cruelly mock him as a trash picker. When the party is cornered by a pack of Hell Hounds, Zoid uses a paralysis spell on Ein and seals the exit with rocks, leaving him behind as bait to die a painful death.

Abandoned and betrayed, Ein leaps from a crumbling ruin within the dungeon in a desperate attempt to end his suffering. Instead of dying, he survives the fall thanks to the intervention of a mysterious power at the bottom of the abyss. There, he encounters Yuri, the spirit of a massive World Tree, and her fierce guardian, the sage Ursula. After Ein is nearly killed by the dungeon's S-rank monsters, Yuri insists on saving him, granting him a prosthetic Spirit Eye to replace his ruined one. Ursula agrees to train Ein in exchange for his promise to help Yuri fulfill her wish of reuniting with her sister World Tree spirits scattered across the world.

Ein undergoes brutal training that pushes his body far past its limits. He discovers that his Appraiser class is not worthless at all; when combined with the Spirit Eye, his appraisal ability evolves into Super Appraise, allowing him to read enemy movements and even slow their actions to a near halt. He learns to counterattack with devastating force and masters powerful magic from Ursula. After defeating the dungeon's boss and absorbing its Labyrinth Core, Ein returns to the surface with Yuri living inside his eye and Ursula able to follow through her bond with Yuri. Upon his return, he exposes his former party's treachery, clearing his name as Zoid faces public disgrace.

Ein's journey to locate the World Tree sisters takes him across the kingdom. He earns the admiration of Princess Claudia after rescuing her from a monster attack, and she declares her intent to marry him. He gains a valuable ally in Jasper, a guard searching for the World Trees. Ein ventures into a hidden illusion dungeon where he meets Pina, another World Tree spirit, and her guardian Kurohime. After clearing the dungeon, Pina and Kurohime join his party, further enhancing his Spirit Eye.

As his reputation grows, Ein accepts an SS-rank quest from the Guild Master Meegan to slay the Behemoth, an ancient dragon spawned by the Demon Lord. He defeats the massive serpentine creature with a cunning strategy, destroying it from within using an illusion spell and a point-blank magical detonation. Meanwhile, the disgraced Zoid, consumed by jealousy and a desire for revenge, is approached by a dark spirit named Echidna who grants him a Spirit Eye of his own. Transformed into a chimera monster, Zoid attacks Yuri and Pina. Ein arrives with Ursula and Kurohime to defend them, leading to a confrontation with his former tormentor.

Following these events, Ein continues his quest, gaining entry to the Forbidden Library where he meets a mysterious girl. He later travels to the region of Rayshik, where a lord named Nigun conceals a dark secret involving demons. Ein faces enemies including a shadowy entity linked to Nigun and demonic threats like Hornet and Ioana. Through each challenge, Ein overcomes overwhelming odds not just through his growing power, but through his inherent kindness and newfound courage. His gentle nature draws allies and admirers to him, and his journey ultimately focuses on protecting those who believe in him while continuing to fulfill Yuri's wish.