From a mindless grey jelly, Rusty's consciousness ignites. He awakens in a dungeon as a clattering, rusted suit of armor. His first encounter with adventurers ends in brutal dismemberment, but his unique nature allows him to reform. Learning from failure, he evolves into a strategic predator, using dungeon monsters as fodder and poison leeches as weapons. He slays his first human—a treacherous adventurer—and realizes that to grow, he must embrace his hybrid nature, culminating in his first evolution into a Corroded Twilight Bronze Armor, a leap that shatters the dungeon's programming.

Pursued by superior adventurers, Rusty plummets into a subterranean abyss. There, he discovers a new world of insectoid monsters and forges a bonding contract with a tiny, silvery ant he names Gleam—his first true ally. Together, they carve through the underground ecosystem, dismantling ant queens and centipede matriarchs. Gleam's magical prowess complements Rusty's tanky resilience, but their partnership is shattered when a party of adventurers captures Gleam and drags her to the surface. Rusty, now a Twilight Iron Armor, is left alone, his metallic heart burning with a new purpose: rescue his friend or burn the world trying.

Rusty's rampage through the dungeon, slaughtering monsters and absorbing their essences, forces the Dungeon Heart to see him as a threat. It revives the Floor Boss to stop him. In a climactic showdown, Rusty overpowers the giant knight and stands before the heart. He doesn't shatter it—instead, he channels his chaotic light and dark energies into its core, overloading its control and gaining the title Unbound. Severed from the heart's influence, he "dies" to the dungeon's eyes. Now free, Rusty dons the mummified body of a slain adventurer named Albert, using the Possession skill to walk among humans. He is now a monster disguised as a man, entering the city to infiltrate the guild and save Gleam.

Under the guise of Albert, Rusty navigates the city's underbelly, joining the Thieves' Guild to amass gold. He clears E-rank dungeons, robs unsuspecting parties, and races against time. At the grand auction, he wins Gleam with a bid of 25 gold coins, but victory is fleeting. A blue-robed mage and his mercenaries ambush him, demanding the ant. In a desperate gambit, Rusty uses himself as a decoy, tossing Gleam to freedom while his body is "killed." Now, his helmet is in the hands of a sinister cult, and Gleam—with a disembodied Aburdon—must stage a rescue, setting the stage for a heist that could end in his true death.