In a quiet corner of the city, four-year-old Kun Oota has lived a spoiled life as an only child with his parents and the family dog, Yukko. But when his new baby sister Mirai is brought home, his simple life is thrown upside-down; suddenly, it isn't all about him anymore. Despite his tantrums and nagging, Mirai is seemingly now the subject of all his parents' love.

To help him adapt to this drastic change, Kun is taken on an extraordinary journey through time, meeting his family's past, present, and future selves, as he learns not only what it means to be a part of a family, but also what it means to be an older brother.

[Written by MAL Rewrite]

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)

The movie is a compilation of episodes 9-13 of the 2017 television series. Riko and Reg descend to the third layer where Riko has her first experience of the Curse. They descend to the fourth layer where Riko's arm is injured by an Orbed Piercer and Reg tries to save her. Nanachi comes to their aid and saves Riko's poisoned arm. In return Nanachi asks Reg to kill her immortal companion Mitty. Nanachi then joins Riko and Reg in their quest to reach the bottom of the Abyss. (Source: Wikipedia)

Continuing their perilous descent down the Abyss, Riko, Regu, and newfound friend Nanachi reach the Abyss' fifth layer, The Sea of Corpses. Upon arriving at the research station known as Idofront, the main trio encounter the mysterious Prushka, the alleged daughter of Bondrewd, who leads them to the White Whistle responsible for Nanachi's dark past. Despite the welcoming appearances of Idofront's residents, Nanachi warns the young adventurers that things are not always what they seem. With the only route to the sixth layer shrouded in mystery and Bonedrewd's schemes awaiting them, what sacrifices must be made in order to continue the journey down to the bottom of the Abyss? [Written by MAL Rewrite]