Yūko begins as a typically talkative schoolgirl whose behavior shifts dramatically, becoming withdrawn and unusually quiet. This culminates in her complete absence from school, prompting her concerned friend Rie to visit her home. There, Rie finds Yūko's parents distressed and actively trying to eliminate a slug infestation surrounding their property.

Yūko is discovered hiding in her bedroom, wearing a face mask and exhibiting extreme reluctance to speak or open her mouth. Her mother explains that medical examinations only identified unspecified mental health concerns. Yūko's refusal to let anyone see inside her mouth creates tension during the visit. Rie later connects this behavior to a deep-rooted childhood phobia, recalling an elementary school incident where classmates used slugs from Yūko's backyard to frighten her, establishing slugs as one of her greatest fears.

Yūko's affliction progresses into a grisly physical transformation. The next day, her visibly deteriorated condition causes her mother to panic. The source of her muteness is revealed: her tongue has mutated into a living slug.

Efforts to reverse this condition prove futile. Surgical removal fails as the slug-tongue regenerates completely. Applying salt directly into her mouth also fails when Yūko forcibly expels it. These interventions leave her unable to eat, causing severe physical decline and sickness.

Faced with no other options, her parents bury Yūko within a bathtub filled with salt. She does not resurface. Her parents retrieve only her empty clothes from the salt. Yūko herself has vanished. Subsequently, her severely salt-shrunken body is discovered, reduced to a tiny size. The slug remains alive and emerges from her mouth, now carrying Yūko's shrunken head attached to its body like a snail's shell. Despite this horrific metamorphosis, Yūko is still alive in this final state; she opens her eyes to convey profound sadness before the slug crawls away with her.