Katie learns the rules of her new existence through sheer physical mutilation. After testing her limits by punching a giant, moss-eating beetle to death and eating its raw flesh to see if she can starve, she ventures out of her starting cave to find a water source. In the dark cavern, she hears a low growl and encounters a pony-sized, black-furred wolf with massive fangs. She runs for the safety of her narrow crevice, but the beast catches her at the entrance, biting her leg clean off above the knee. She drags her bleeding, mutilated body back to her cave, desperately tying strips of her nightie around the stump before slipping into unconsciousness. She wakes up fully healed with both legs intact, but the sight of her torn nightie on the floor and the half-eaten beetle corpse prove the world does not reset when she dies. Her skills and items persist across her deaths. Seeking water again, she finds a beautiful silver tree growing in a crystal-clear pool, but her first taste of the sweet liquid triggers a poison tolerance notification. The tree is a passive predator, and the water is a paralyzing nectar. Wriggling roots wrap around her ankle, drag her to the bottom, and pin her flat on her back. As her lungs run out of air, she is forced to swallow the numbing liquid, letting her heart paralyze. This second death confirms her immortality is absolute. She begins using the murderous tree as a tactical reset button to heal her wounds and farm resistance skills. To escape her starting area, she crafts makeshift armor from giant beetle shells and a spiked shield. She baits the wolf pack, impaling them one by one on her shield spike in a desperate, bruising battle. She secures her first reliable water source, but the dark tunnels ahead lead her straight into a web-covered corridor where a far more intelligent predator waits.

The web-filled tunnels bring Katie face-to-face with the aranea volito, a species of giant, pale grey spiders that can teleport along their silk threads and spit highly corrosive acid. During her first encounter, a spider outmaneuvers her, dropping a section of the ceiling web onto her head and cocooning her from the feet up. Instead of screaming in terror, Katie bursts into hysterical, mad laughter, completely baffling the sadistic predator, which expected her to beg for mercy before injecting its digestive enzymes. After waking up from her third death, she returns to the white-bricked shrine room only to find herself ambushed by eight spiders. They coordinate their attacks, spitting balls of yellow acid that melt her makeshift armor and splash into her eyes. Relying on her newly evolved pain resistance, she weaponizes her mad laughter to agitate the spiders, driving them into a mindless berserker rage that breaks their strategic coordination. She drops her shield, draws her spider-claw knife, and stabs them in their eyes as they swarm her. Though she slaughters all eight, her legs are shredded, and she accidentally shoves her hands into a puddle of acid, melting her fingers into bone stubs. To escape the agonizing paralysis, she uses her remaining strength to slit her own throat. This victory clears the shrine, activating her first class selection: the unique Princess of Undying Laughter. This class grants her the item box skill, allowing her to store weapons and transfer her equipped armor across respawns. She uses her new spider-claw knives, which are immune to the sticky silk, to harvest the webs and upgrade her gear. But her victory is short-lived as the scent of blood draws a massive, writhing swarm of centipedes up from the lower levels, threatening to overrun her newly cleared sanctuary.

The descent into the lower caverns triggers an invasion by the chilopoda, a mindless, writhing sea of multi-legged centipedes. When Katie fails to defend the shrine barrier, the swarm overwhelms her, paralyzing her with venom. They drag her into a pitch-black breeding chamber, gluing her to a wall with hardening amber fluid next to a row of decaying, exploded monster corpses. The massive, psychic brood-mother centipede, sporting a giant exposed brain, lowers its head and forces a lumpy, egg-filled tentacle down Katie's throat, turning her into a living incubator. The horrific physical sensation of the parasitic eggs wriggling inside her stomach pushes her disease resistance to its absolute limits. Realizing she is about to be eaten from the inside out by hatching larvae, Katie uses her item box to cut herself free and launches a desperate suicide attack, driving a spear deep into the brood-mother's exposed brain. In response, the psychic monster unleashes powerful mind magic, completely overwriting Katie's memories and brainwashing her into believing she is a loyal, deformed centipede offspring. Only her pre-activated trigger respawn skill saves her, plucking her soul back to her cave and breaking the illusion. Furious at having her mind violated, Katie returns with a harvested sac of corrosive spider acid. She climbs the brood-mother's face, pours the melting acid directly onto its exposed brain, and stabs it repeatedly with her daggers, ignoring its telepathic pleas for mercy. She successfully purges the psychic tyrant, but her victory alerts the neighboring fox-kin tribe, who view her as a dangerous, shapeshifting threat. The guards at the checkpoint tighten their grip on their spears, preparing to capture the immortal intruder.

Katie's attempt to contact the fox-kin beastmen ends in disaster when their monster farmer, Ja'yakril, tries to force an enslavement collar onto her neck. She uses her item box to steal their weapons and collars, then slits her throat to escape, accidentally triggering a brutal civil war within their town. The arch-priestess, who verified Katie's divine blessing of immortality, goes on a bloody rampage against the warrior caste for torturing a servant of the Goddess, ultimately destroying the town's barrier and leaving it vulnerable to a devastating plague of undead blight. Amid the ruins, the brilliant arch-mage, Mru'walyn, uncovers a terrifying truth: their entire universe is a fake, fifteen-kilometer cylinder created only a week prior to serve as a trial for Katie. Realizing his soul-less people will be erased the moment she completes her quest, Mru'walyn uses forbidden soul magic to capture her, attempting to wipe her memory of the surface world to freeze the quest indefinitely. Katie escapes by suffocating herself with hardening fluid and retreats to the catacombs, where she teams up with DupliKatie, an intelligent zombie clone born from her own blighted corpse. Together, they confront the animated armor of a mad ancient king that controls the catacombs' shifting maze. While DupliKatie distracts the boss, Katie uses a stolen teleportation dagger to banish the armor directly to Mru'walyn. When the arch-mage teleports down to finish her, DupliKatie bisects him with a massive two-handed sword, ending the fox-kin's desperate bid for survival. To prepare for the dangers of the fourth floor, Katie unlocks her second class, Aberrant Monster Tamer, which mutates her body with spider spinnerets and chitin skin, but grants her the power to descend into the pitch-black depths.