The film centers on the Hosino Tribe, a large colony of cockroaches living inside an apartment owned by a human named Mr. Saito. Saito, who lives alone and is notoriously messy, unknowingly creates the perfect playground for the roaches, who come to see him as a generous benefactor while luxuriating in a life that defies their traditional, cautious ways. The tribe is led by a roach known as Sage/Professor, a figure who dispenses advice and claims to negotiate on the group’s behalf. Long ago, the roaches waged the Hosino War against humans; the humans left when the older Hosino clan departed, and Saito arrived to oversee the peace. The roaches still mark the moment with Armistice Day, a memory that underscores their fragile truce.
Two young roaches, Ichiro and Naomi, are engaged and planning a wedding, though Naomi harbors doubts about the union as Ichiro struggles with a winged limitation that keeps him from flying. Their quiet plans are upended when a roach from a distant tribe, Hans, arrives badly injured, and Naomi finds herself drawn to him. After healing, Hans returns to his own people, but Naomi secretly follows, driven by a desire she barely understands.
Naomi’s discovery leads her to another apartment run by a woman named Momoko. Momoko’s faction is at war with Hans’ militaristic group, with food as the battleground and insecticides as the weapons. Naomi seeks help from Momoko, yet the encounter ends in hostility, as Momoko tries to stomp on her. The two factions continue to clash, and Naomi and Hans are reunited for a period of three weeks, living together in a fragile peace that feels almost hopeful.
As tension rises, one evening, Saito and Momoko cross paths from opposite sides of the complex, and the roaches remain unaware of each other’s presence. During a visit, Naomi is forced to hide in Momoko’s purse and ends up back at Saito’s apartment after a harrowing night in a roach motel, a sequence that strains her memory. Ichiro and Naomi are eventually reunited, and Naomi confides that she has amnesia and does not recall the events that led her away. The couple proceeds with their wedding, a moment that promises renewal even as danger looms.
The ceremony is disrupted when Momoko launches an attack and kills many of the attendees. A funeral for the fallen roaches follows, and Sage announces that he will discuss the outbreak with Saito, though the next day he is found dead, pinned to a dart board. Soon after, Saito and Momoko push to wipe out the roaches with mass extermination. The Hosino tribe, caught off guard after generations of relative peace, suffer severe losses. Naomi reveals to her grandfather that she is pregnant, unsure whether the litter is Ichiro’s or Hans’s, with her grandfather suggesting it is likely a mix of both.
Ichiro learns from two elderly roaches that much of what he was told about the Hosino was a lie, and that Sage did not know the truth because he had hidden in the cellar during the war and had never actually met Saito—left to infer the peace. The elderly roaches, who remember life before the ceasefire, teach the others to forage and steal dropped food, but hunger soon gnaws at them again. When Hans’s tribe arrives to invade, they discover the Hosino have little to eat and, after weighing their own survival, agree to help, even as both Ichiro and Hans acknowledge their love for Naomi and decide to settle their conflict after the battle.
Saito and Momoko intensify their plan to kill all roaches, setting off a gauntlet of bug bombs and sprays. In the ensuing battle, Hans is killed when a book falls from above, foiling his rallying cry. Naomi and Ichiro survive the onslaught, though Ichiro is gravely wounded and Naomi appears to die in the inferno of insecticides. Naomi miraculously lives, her supposed immunity proving real, and she escapes with a vision of a future free from humans.
The epilogue shows Naomi with a vast litter of children who seem immune to insecticides. Some of the offspring resemble Ichiro and Hans, implying that both men contributed to the next generation of roaches. The film closes on a note of tentative hope, showing a world where the roach families endure and adapt, finding strength in their shared heritage and the possibility of a future that honors both loyalty and love.
