The movie is a compilation of episodes 1-8 of the 2017 television series with new scenes added for the introduction. It covers the period from when Riko descends into the Abyss with her robot companion Reg, reaching the second layer where they meet the White Whistle Ozen who reveals information about Riko's mother. (Source: Wikipedia)

After bonding over a tragic loss, the long-suffering Nanachi joins Riko and Reg on their journey into the depths of the Abyss. Awaiting the children is the Sea of Corpses—the Abyss's fifth layer, and the deepest level from which a traveler can return without losing their human form. The masked sadist Bondrewd stands between the children and the rest of their adventure. Bondrewd's horrific laboratory serves as a final checkpoint for those wishing to traverse deeper into the Abyss, and the sociopathic scientist has no desire to allow Riko's party to pass through at no cost. Deeply scarred by Bondrewd's impact on their childhood, Nanachi is engulfed in turmoil over his resurgence in their life. Bondrewd's only apparent weakness is Prushka, a brash child who claims to be his daughter. Riko, Reg, and Nanachi befriend Prushka and work with the girl to overcome her father's machinations and breach the Abyss's sixth layer.

The Abyss—a gaping chasm stretching down into the depths of the earth, filled with mysterious creatures and relics from a time long past. How did it come to be? What lies at the bottom? Countless brave individuals, known as Divers, have sought to solve these mysteries of the Abyss, fearlessly descending into its darkest realms. The best and bravest of the Divers, the White Whistles, are hailed as legends by those who remain on the surface. Riko, daughter of the missing White Whistle Lyza the Annihilator, aspires to become like her mother and explore the furthest reaches of the Abyss. However, just a novice Red Whistle herself, she is only permitted to roam its most upper layer. Even so, Riko has a chance encounter with a mysterious robot with the appearance of an ordinary young boy. She comes to name him Reg, and he has no recollection of the events preceding his discovery. Certain that the technology to create Reg must come from deep within the Abyss, the two decide to venture forth into the chasm to recover his memories and see the bottom of the great pit with their own eyes. However, they know not of the harsh reality that is the true existence of the Abyss.

After surviving the brutal challenges of Idofront, Riko now possesses a White Whistle, allowing her to descend into the Abyss's sixth layer—The Capital of the Unreturned. Alongside Reg and Nanachi, Riko begins to explore the uncharted domain, where the ruins of the promised Golden City are located. As the trio starts to adapt to the harsh environment, they soon encounter dangerous creatures and treacherous landscapes. Their expedition leads them to a village inhabited by strange beings known as hollows. Despite the creeping sense of unease that welcomes them, the three venture onward to uncover the mysteries of the settlement and long-lost legacies of the forgotten adventurers who once descended into the horrors of the unexplored Abyss.