In the picturesque steel mill town of Mifuse, tucked away in the depths of rural Japan, two brothers, Akimune Kikuri and Tokimune Kikuri, strive to make a living amid the rusting remnants of machinery. Their son, Masamune Kikuri, spends his days preparing for an upcoming entrance exam with friends, when suddenly, a catastrophic explosion on January 1st, 1991, rocks the mill, casting the town into an everlasting winter of stark desolation and cold.
As inhabitants grapple with the overwhelming isolation, Mamoru Sagami, a devoted worker and self-proclaimed priest of the Mifushi Shrine, asserts that this tragedy is a form of divine punishment, an act of retribution for their past transgressions against the sacred mountain. Sagami mystically claims that they are now entrapped by the very steel mill they once relied on, which has morphed into a "sacred" entity. He warns that changes to their situation will only worsen their plight until the gods show mercy.
As the townspeople submit periodic biography ID forms to keep the peace during this surreal existence, strange fissures start to crack the night sky, uncannily reflecting the ceaseless smoke billowing from the mill. Masamune and his friends, seemingly unaware of their bizarre entrapment, occupy themselves with bizarre games that challenge the concept of pain.
The next day, Mutsumi Sagami, a classmate whom Masamune has little in common with, invites him to visit the mill's fifth blast furnace. There, they discover a feral girl on the cusp of adolescence, brimming with untamed spirit. As Mutsumi takes it upon herself to care for the girl, now named Itsumi, Masamune begins to sense the futility of trying to escape their circumstances.
Meanwhile, Akimune, Masamune's father, labors through endless shifts at the mill only to disappear without a trace. As the days blend into one another, Masamune and his companions must confront the eerie reality they seem trapped within—a reality where pain has become a distant memory, and the boundary between fantasy and nightmare grows increasingly hazy.
In a poignant moment, Masamune finds himself at the mill, engaging in a ritual of care as he feeds Itsumi while naming her. Their tranquil scene is interrupted by Sagami and Tokimune, both exuding an air of mystery. Sagami claims Itsumi is a divine presence, one who will ultimately be forgiven by the sacred machine. This is met with skepticism from Tokimune, who recalls their father's cautionary words, hinting at deeper truths, while Masamune attunes himself to the whispers of the past.
Masamune's visits to the mill become a routine spurred by Mutsumi's insistence. Here he encounters Yuko Sonobe, a classmate who inadvertently steps into his life with news about school matters. Their lives intertwine again during a revealing moment at Mutsumi's home, where it emerges that she is, in fact, Sagami's adopted daughter.
As seasons shift, the group—Masamune, Mutsumi, Sonobe, and their classmates—find themselves embarking on a treacherous expedition through an abandoned railway tunnel, navigating rugged pitfalls. During this perilous journey, Sonobe's impulsive act of creating graffiti adorned with romantic symbols leaves Masamune in awe. However, her hasty retreat coincides with the ominous sky fissures and the unsettling glow surrounding her, marking a crucial juncture in their shared fate.
Gathering at the community center in search of comfort, the townsfolk absorb Sagami's chilling revelation that the factory smoke embodies a "sacred wolf" capable of breaking hearts and sealing the fissures in the sky. Amid this tension, Masamune finds himself retreating to a contemplative silence in the town square, confronting the implications of Itsumi's origins and the cryptic hints from Tokimune regarding her precarious position in their world.
As Masamune cherishes another moment with Itsumi, her happiness clashes with the stark reality where she longs for freedom. He refuses to let the strangeness of their lives deter him as he ventures forth, yet Tokimune's pleas to maintain stillness echo in the background. Visibly yearning for the outside world, Masamune's courage builds, especially under Itsumi's encouragement, pushing him to reach out towards the fissure—an opening to a world that reveals their home as a tormented ruin.
In a breathless confrontation, a briefing session at the community center ignites fervent debates, as Sagami reiterates the notion of their unreal world sculpted by the sacred machine. While doubts bubble among the residents, Tokimune drops the shocking truth: that the surreal reality continues to exist at the cost of Itsumi's return to the mill—sparking unrest among the townspeople.
Amidst this turmoil, the sacred wolf's malevolent shadow looms as it claims lives, including that of Yasunari Senba during gym class. As Itsumi and Mutsumi take sanctuary in Masamune's home, unsettling occurrences escalate. A mystifying vision of a middle-aged figure resembling his father appears before Masamune, conversing with Mutsumi about their past, while she reveals the truth about Itsumi once being Saki Kikuiri, their daughter struck by fate.
As the narrative unfurls, Masamune's resolve intensifies, ultimately leading to a heartfelt confession in his desperate quest for Mutsumi's affection, which ends in a bittersweet rejection. Nonetheless, their connection deepens in a tender moment, cloaked beneath the umbra of unfolding chaos as Itsumi's tears unleash a cataclysm of fissures permeating the fabric of their reality, coinciding with the mill's halt in production.
As confusion reigns over evacuation activities, Masamune receives his father's forgotten diary, uncovering the profound truth about Itsumi—her transport from the real world and his previous life as Mutsumi's husband. The urgency of Saki’s return gains traction, prompting a counterproposal from Sagami—a refusal that lands Itsumi back in the furnace's grip.
As Masamune rallies his classmates to rescue Itsumi, Tokimune embarks on a reckless mission of reigniting the blast furnace, desperately clutching to hope for their impending doom. In the midst of an enigmatic ritual proposed by Sagami, Masamune and Mutsumi guide a reluctant Itsumi onto a train bound for the outside world, orchestrated by Masamune's grandfather, Souji Kikuiri. Their ascent, however, meets an untimely derailment, a chaotic twist to their fate.
In the melee, a fierce rivalry ensues, each friend vying for Itsumi's affections, culminating in a dramatic escape that sees Masamune piloting through the fissure to converge with the real world’s train station. With Mutsumi's assistance, Itsumi narrowly boards the train and, in a poignant farewell, she makes a solemn vow concerning her heart. As the train departs, Mutsumi wrestles with her sentiments while Itsumi embraces her with a shroud of hope that lingers amidst the chaos, ultimately leading to Masamune catching Mutsumi upon their world’s abrupt collapse.
Years roll on until Saki, now blossomed into a young woman, returns to Mifuse, greeted only by the ruins of a once industrious steel mill. This ghost town now stands abandoned—a husk of memories. As she wanders through the remnants, she is drawn to a glass wall inscribed with heartfelt messages, the essence of the cherished past imprinted in that forsaken world. Climbing to the furnace's viewing platform, Saki reflects on the bittersweet connection between Mutsumi and Itsumi, her thoughts adrift in nostalgia.
