Dahlia is stranded in the sewers with the dying bug trader, a massive cave cricket guarding the only ladder out, and no weapons. Eria, her new worm system, tells her to eat the cricket to gain power. She cannot stomach it. Instead, she searches the trash with her eyes and hands, finds six vials of bombardier beetle extract, and takes her father's lesson to heart: there is more than one way to fight. She crafts a bomb from discarded beetle carapace and wax glass, throws it, lures the blind cave cricket with vibrations, and blows it apart from the inside. For the first time in her life she kills a Giant-Class insect, and the moment of victory turns sour when her body spontaneously rejects the cricket's meat. She cannot eat to gain power. Her only path forward is to build her strength, and she carves the cricket's forelegs into bracers that let her sense danger through vibrations. The bracers meld to her forearms and give her a whisper of power, but they do not give her the clean, digestible points that would let her grow fast enough to survive the Swarm. She climbs out of the sewers alone, carrying only her guilt, her bracers, and the ghost of a promise to return for the bug trader's corpse.

Issam and the twins pull Dahlia out of the wolf spider's chase, and she wakes in the shelter to hear the truth: most of Alshifa is dead, the Instructors are gone, and only a handful of students are capable of fighting. They set out for the school to recover Amula and Jerie's Swarmsteel and find the bridge blocked by a D-Rank pine sawyer beetle. Issam tries to execute a clean kill and gets his sword trapped in the seam of its chitin. The beetle almost kills them all. When the group falters, Dahlia's eyes catch a line of shimmering steel thread that runs from her chisel to a gap in the beetle's armor. She dodges its legs, cuts its chitin apart, and with Issam's final blow, the beast dies. Eria tries to assign her a class, but nothing fits, she is given the temporary mantle of Hemiptera and nothing more. The anomaly, Eria admits, points at something fundamentally wrong with her body. The only clue is her growing sense that she knows what her class really is, and that she is not willing to name it. They escape the school by crafting a glider from giant robber fly wings and ride it over the ruined town, hunted by a lightning-wielding hornet.

The glider crashlands into the Sarowan Garden, and Dahlia spends a night of rest before insisting on returning to her father's house on the northern hill. What she finds is a carnage: the Old District is littered with giant insect corpses that have been torn apart by claw marks too precise for any beast. Her father, Doctor Sanyon, has mutated past the point of recognition. His back is bent, he has grown insect arms and a needle for a tongue, and he attacks her. The fight that follows is the most painful sequence of her life. Raya and Amula come to help, but they are broken, and Dahlia fights alone. She follows the steel thread and slices through his chitin, his legs, his heart, and he finally falls, smiling at the empty air and calling her "Eria," her mother's name, believing he is already dead. She learns the full truth: her mother was a surface outsider whose compulsion to eat bugs eventually drove her to mutate into a black assassin bug, and Sanyon had killed her to protect their daughter, then injected himself with her blood to try to find a cure. He has been turning for two years. Dahlia's final act is to pry the chitin shards from his claws and leave, carrying them with her, knowing she will have to live with what she has done and who she is.

Raya and Amula rescue the group, and the survivors learn the terrifying scale of the surface war: seven fronts, a hundred million humans, and a sky sealed by Mutant-Class titans. The lightning hornet still sits at the Bazaar, and Dahlia devises a plan to lure it into a firefly light and bombardier bomb trap. The trap works, Issam severs its abdomen, but the creature reveals its true form as a photuris firefly that regenerates and turns the trap against them. Raya charges alone to buy her five minutes, and he dies in that explosion. Then Issam dies covering her from the final lightning blast, leaving her his handmade pocket watch as a love declaration and telling her to make her own destiny. With the creature spent, Dahlia rises, and the steel thread guides her through a furious dismantling. She tears the firefly apart with her own hands, and in the rubble she begins to eat it, unable to stop herself. Eria assigns her the class that fits at last: the extinct and forbidden Assassin Bug. She gains two extra arms, a T1 mutation called Swarmguard Deity, and a Swarmblood Art that shows up as corrupted text. The system finally unlocks, but the cost is everyone she loved, and the girl who was once terrified of bugs is now something the world forgot was possible.