The episode starts off with flashbacks of Reg's and Riko's adventures up to the ending of episode 12. We then see Nanachi dragging Reg back to her hut (Reg had fired his Incinerator in the previous episode's ending). We then start a flashback showing the early happenings of Nanachi and Mitty and how they got into their current position.
It starts with Nanachi at what seems to be an orphanage. Nanachi is lying down reading a book, thinking to themself. Children in the hallway outside their shared room leave to go outside; Nanachi follows. As they gather, Nanachi is wearing tattered cloths; a boy comments on their bad smell.
A White Whistle, Bondrewd, stands in front of the children and explains to them he is a chief and tells them, "I've come here in search of children suitable for forging a path to the netherworld's next era". He asks the brave children to step forward.
We see Nanachi looking into the Abyss from a building in Orth that looks like Reg and Riko's old orphanage. Two kids are denying Mitty the chance to work with them. Our first look at normal girl Mitty shows she has long red hair and red eyes. She wears only a bra and loose underwear (most of the other children have the same outfit). Nanachi thinks to themself, "So we're not the only ones, then, huh?"
In an unknown place, the children are gathered in front of a vessel to begin their descent. Bondrewd explains that they will be guided by other cave raiders and their descent is ~13 kilometers and that they will be at the bottom of the fifth layer and that they will strive to unravel the final mysteries of this world.
During their descent, the children wrap up in blankets—some are even sleeping. Nanachi is eating delving rations, which they don't like. Mitty and Nanachi have their first interaction: Mitty gets in Nanachi's face asking about Nanachi's ambitions, then immediately explains her own (to be a White Whistle). Nanachi is annoyed. Nanachi explains they don't care what they do as long as they come to the Abyss, and that before this, they dug food from garbage to eat. Mitty explains of the treasures to find; the two agree to be partners.
The two read from a book called "Capital of the Unreturned"; it's written in nether glyphs, which Nanachi can read, but most can't. A monster can be heard outside the vessel the children are in, which scares them, but Nanachi looks generally unconcerned.
They arrive at their destination. A voice from an intercom says that they will suffer the strains of ascension if they climb more than 10 meters. The children gather at the end of a tunnel to find a beacon, a beacon that looks similar to the one during the opening. They look and hold hands as Nanachi appears nervous. Bondrewd shows the children to their room.
Mitty asks about the "Abyssal Faith". Nanachi explains "in a nutshell", "When a life is lost in the Abyss, the soul is said to return to the bottom of this planet and then the soul changes form and departs on a journey to someone who has wished for life. The pit replaces God and that its a comfort to cave raiders." Mitty praises Nanachi. Over time, Bondrewd comes back and asks for a child to come out until only Nanachi and Mitty are left. Mitty wishes for them to go out together. Mitty and Nanachi are sleeping together; Bondrewd calls for Mitty and tells Nanachi to be patient.
Nanachi sneaks out of the room and explores. They overhear a man and Bondrewd talk about how they killing children is inhumane. Nanachi runs and searches for Mitty. Nanachi finds Mitty inside a clear cylindrical container. Bondrewd catches Nanachi and explains their perfect timing in that it's their "turn". Nanachi is placed inside another container by Mitty; Bondrewd explains that they are elevators that can go into the sixth layer quickly. The destination is a dead end but is perfect for research on human subjects. The sixth layer's Curse: "Death, or loss of one's humanity". The two are dropped into the depths. They land on a pile. Around them are mutants who have lost their humanity. Mitty assures Nanachi about her fate. They ascend. Nanachi turns into the present form Nanachi. On Mitty's side, she's bleeding and screams that it hurts. Nanachi expresses their friendship to Mitty as Mitty mutates. Mitty asks Nanachi to kill her.
Bondrewd opens the elevators and expresses his interest in Nanachi. Nanachi is distraught. Nanachi is drawing a crude picture of Mitty. Bondrewd says that they should be helping with research. He says that Mitty lost her humanity but also is incapable of dying. Bondrewd has tried to kill Mitty nine times, but she won't die as she grows her limbs back.
Nanachi escapes with Mitty. Nanachi roams across a snowy land. Fast forward to Nanachi establishing their hut. Nanachi tries to kill Mitty many time with no success, but instead causes her suffering. A cave raider is bleeding and Nanachi takes him in. Nanachi is unable to save him, so they kill and bury him.
After Reg wakes up (back in the present), Nanachi explains he has the perfect weapon, that Mitty won't grow back with him using his Incinerator. They say that Mitty has totally lost her humanity and that her soul is imprisoned in her body. Reg asks to think about it. Nanachi apologizes for asking Reg to do such a thing. Reg contemplates if Nanachi will kill themself after he kills Mitty.
Mitty is chewing on Riko; Reg stares. Nanachi says they'll make a meal; Reg insists he do it. Reg fails at making anything good; it's worse than Nanachi's creations.
Reg is cleaning Riko. Nanachi is reading to Mitty. Mitty comes out of her place; Reg sees Nanachi asleep. Reg wonders if it's okay to leave Mitty alive.
Reg stands by Nanachi as they're fishing. He asks them if Mitty is unhappy. Nanachi predicts his next question is about the necessity to kill Mitty. They explain that Mitty can live forever but they can't. They say that when they die, Mitty will be alone for eternity. If Mitty is stabbed, it hurts her, and she'll be forced to suffer forever. They ask again for Reg to kill Mitty. Reg asks for a promise that Nanachi not kill themself, Nanachi promises, and Reg agrees to kill Mitty.
Nanachi sets Mitty outside in a pile of stuffed animals. They talk to Mitty. Reg prepares his weapon. As Reg's weapon charges, Nanachi calls for him to stop. They run over to Mitty and cry over her one last time. They walk back and tell Reg to do it. Mitty is destroyed. They turn their back, not looking at Reg's work until he's done. Nanachi and Reg cry.
We see Riko's mother, Lyza, telling Reg not to be sad; she offers him food. Reg wakes up. Riko is awake again. Riko provokes[clarify] her hurt arm. Riko and Nanachi have been cooking. Nanachi asks Reg to stop Riko from rubbing them. Reg wants to join in. Riko makes a meal; Nanachi loves it, but pretends it's only okay. Riko asks aloud if someone else lives in Nanachi's hut; this shocks Reg and Nanachi into silence. Nanachi asks how she knows: Riko has to ask that herself. She then remembers a dream.
She explains she was in between two rocks. They kept squishing her with much pain. Riko says she didn't remember who she was nor how to speak, that she was very scared and was crying. She then heard another girl crying and says she sounded scared. Riko/Mitty wanted to tell Nanachi that it would be alright.[dubious] She smelled smoke and the crying stopped. Riko says she could see the girl and make out her profile. The girl turned her back to Riko. Riko says she looked like a cave raider. The girl is regular Mitty. Riko then remembers and then wakes up.
Reg explains that Riko was once dead. Reg tries to use this tie[clarify] as an explanation. Nanachi says it's alright.
The company leave to a hot spring. Reg helps undress Riko; the two get in. Riko notices Reg's (erect?) penis; Reg is heavily embarrassed. Nanachi refers to the time Reg blew air into Riko's lungs and spins it off as Reg kissing Riko, making Reg more embarrassed. Nanachi asks Mitty silently to wait for her a bit longer.
Back at the hut, Nanachi and Reg remove the parasitic water shrooms from Riko's arm and she is in much pain, urinating a second time. After the operation, Riko is trying to move her hand. She only is able to twitch her thumb. Reg moves in and apologizes to her for the whole situation. Reg and Riko fight to take the blame. Nanachi is listening from a distance away, cringing. Riko asks Nanachi to come with them on their adventures.
The company release a message balloon and we see its progress to Orth. They then go and prepare their things to leave. Nanachi looks back at her hut for the last time.
Lastly, Bondrewd is surprised that Mitty's life signature has disappeared; he offers his gratitude to Nanachi and wishes to meet her again.
- Volume 3, Chapter 23: Dreadful Experimentation
- Volume 3, Chapter 24: Liberation of the Soul
- Volume 4, Chapter 25: Survive from Darkness
- Volume 4, Chapter 26: A New Start
- Nanachi and Mitty escaped from Idofront, the Forward Operating Base due to Bondrewd's dreadful experimentation
- Bondrewd (present)
- What's in the package?[clarify]