OHOHO IM GLAD YOU ASKED >:3
so basically, eight died during the events of octo expansion, and three can't remember why but she is absolutely not okay. but three's also lost a lot of her memories from what happened in the metro, so she's got like a vague idea of the love of her life being ripped from her and she's going a little crazy
then eight appears in her room as a ghost and asks for her help and three's like. fuck that man you're not real. but eventually comes around and decides to help her. eight's deal is that she's a ghost/soul thing (it's a very messy concept that doesn't make a lot of sense but pretend it does) but she lost a lot of her memories both when she ended up in the metro and when she died. but if she can get her memories back and find her body, she'll be able to come back to life. so they go and explore the metro, fighting sanitized octos, finding eight's mem cakes and being really gay! (also since eight's a ghost she and three can't touch each other and the mutual pining is STRONG) but also eight seems to be hiding a lot of details, like the exact nature of her death and the gaps in three's memory...
i've got a couple chapters written already and i plan on posting it when i'm done (tho i've considered posting it as i write so ppl can watch me descend into madness in real time)