The story begins with Eiren, a young boy with the Tamer profession, being unceremoniously kicked out of an S-rank adventuring party by its leader, Zack. Zack, a boastful and aggressive individual, deems Eiren useless in combat, despite Eiren's companion wolf, Ran, possessing an unusual sensitivity to danger that warns the party of approaching monsters. Asuna, a kind and beautiful Magic Knight and a childhood friend of Zack, defends Eiren, highlighting his crucial non-combat roles like scouting and carrying supplies. She also reveals Eiren's motivation: to earn money for his ailing grandfather's medicine, which requires a rare Phoenix Feather. Zack dismisses this as an impossible dream for a "pauper."
During a dungeon exploration, Eiren experiences headaches, hearing strange voices related to "spirit cores." Asuna, who clearly cares for Eiren, comforts him and suggests they go to a hot spring after the dungeon. Eiren's continued presence in the party is largely due to Asuna's unwavering support.
The party progresses through an S-rank dungeon, with Zack often being loud and reckless, attracting monsters. Ran's abilities to detect enemies and deter them with its howls are instrumental in their relatively smooth progress, though Zack and the other party members fail to acknowledge this. They reach the boss room, and Asuna instructs Eiren and Ran to wait outside for their safety. Eiren feels like a burden, but Asuna reassures him.
Inside the boss room, Zack and the party are overwhelmed by Cyclops. They flee, with Zack callously ordering the Sage, Dina, to use Eiren as bait. Dina paralyzes Eiren, and Zack abandons him, along with Asuna, who refuses to leave him. Asuna bravely fights the Cyclops alone, but is eventually incapacitated. Ran attempts to defend Eiren and Asuna but is brutally killed by a Cyclops.
In his despair and facing his own death, Eiren awakens a hidden power. A Phoenix Spirit Core within him hatches, transforming him into an Elemental Master. He gains immense power, capable of wielding Phoenix-based abilities like the Phoenix Arrow and Phoenix Blow, effortlessly defeating the Cyclops. He also discovers that Ran was a divine spirit, a Fenrir, and that his own body hosts a Phoenix spirit, Karen. He uses his newfound healing abilities to revive both Ran and Asuna.
Meanwhile, Zack and his remaining party members, stripped of their spirit cores and thus their powers, are struggling against a horde of Kobolds. They realize their reliance on Eiren's presence, which had somehow amplified their own powers through the spirits' favor. Zack, however, refuses to admit his mistake and blames Asuna for choosing Eiren over him.
Eiren, now a powerful Elemental Master, forms a new party with Asuna and Ran. He uses his abilities to quickly travel to his grandfather's village and heal him with the Phoenix's power.
Back at the Adventurer's Guild, Zack and his party face consequences for their actions. Eiren reports the events truthfully, leading to Zack being fined heavily and forced to sell his assets. Zack's attempt to attack Asuna is thwarted by Eiren, who has gained the ability of precognition. Asuna declares she is leaving Zack and forming a party with Eiren. Zack is arrested for attempted assault. Eiren, Asuna, and Ran begin their new life together, while Zack and Dina face ruin due to their loss of spirit power.
