Firstly, the basic premise:
The house is a kind of purgatory that isn’t exactly bound by time. The children who live there were all killed and abused by men in tragic circumstances. There’s an element of pre-destiny, whereby the house draws in the men who have harmed these young girls, so the children can enact vengeance on them. In the process, they extract fluid from each man and collect them as the “nectar of dead men,” as well as taking their veins/muscle chords (whatever those red strings are, I’m no biologist lol), and tie them into a tapestry. And Henry is next in that line of men who need to be punished.
What did Henry do?
Like @hcamille said above, my biggest source of confusion is around Henry, and what he did. It’s clear that the multiple stories he tells in the dinner scene correspond with the other girls: Marlow was the one killed raped by her father and killed by her mother, Bernie was obviously the deaf girl who was attacked, “A1656” was the girl in the concentration camp, and Squirrel was the one who was chased through the forest in the 1800s. Henry’s crime was hurting his lab partner, I guess.
But why does he say he did ALL those things, and who is the lab partner that he hurt? At first I thought it was Squirrel, because of the way she “takes the lead” on guiding Henry through his guilty past. But that doesn’t make sense with the years given - I’m pretty sure they said Squirrel was chased through the forest in the 1800s.
But I think @hcamille’s theory makes sense, that the multiple stories he tells maybe came from having drunk those men’s “nectar.” But whatever the literal explanation (or lack thereof) I do think it’s supposed to be symbolic of men throughout history abusing young women, and how all these men are reflections of each other; they’re all the same, underneath.
What causes the girls to arrive at / leave the house?
At first, when I thought that Squirrel was Henry’s lab partner, I figured that when each girl’s respective abuser is killed, she gets to move on. But that doesn’t make sense if Squirrel isn’t the lab partner. Plus, it also doesn’t make sense if Hank drank the nectar of the other girls’ abusers, because then they would’ve had to have been through the house already - and the other girls still haven’t moved on.
That, combined with the fact that Hank’s lab partner isn’t present in the house, made me lean toward this theory:
Could it be that the girl at the end is Hank’s lab partner? If the house is a safe haven for the girls, maybe her spirit couldn’t arrive UNTIL her abuser had been taken out.
The old lady:
I think it’s heavily implied that the old lady is Squirrel all grown up. She’s always wearing the same thing as Squirrel, and she moves similarly (when we first see her, she climbs out of the wall, and we know that Squirrel likes to get inside the walls too). Metcalf has a line about how ghosts are the reflections of our future selves, and because the house exists in a blurred state of time, the “future shadow” of what Squirrel would’ve become begins to appear, and she too gets to move onto the afterlife in the end, at the same time that young Squirrel does.
But if Squirrel isn’t the lab partner, why is she the one that gets to move on?
I don’t think this is explained, but I think we’re meant to assume that there’s a natural ebb and flow to the way the girls come in and out. They knew that with a new man arriving, a new girl would be coming. So Squirrel maybe knew, somehow, that it was her time. She was ready to leave, so she “took the lead” on Henry’s downfall, and maybe that’s what allowed her to move on?
The Boy (Mr. Man)
This one might be leap, but I think (??) it’s pretty heavily implied that the boy in the house is Henry’s younger self? Again, the house exists in a blurred state of time. And in the bedtime story scene, the play draws clear parallels between Henry and the Boy/Mr. Man. He remembers the bedtime story, presumably from when he was that age, and it haunts him. And afterward, they play a sort of “peek-a-boo” where their movements mirror each other. Also, Marlow (Caruso) says that the Boy has had many different names, which could be interpreted as the Boy always being the younger self of whichever man is currently in the house.
Max and Raleigh
Because Max (Maslany) came with Hank, and because she gets along with the kids so well, she’s essentially getting prepped to replace Raleigh (Metcalf) as the new mother of the house. And it’s implied that this was pre-destined, in some way.
Max arrives, and realizes she has a connection with the kids. She grows to love them and empathize with their tragic circumstances, and so decides to take over Raleigh’s role as Raleigh takes her leave. Max wants to leave too, at first but when she sees the “new girl” enter at the end, she has a change a heart. Upon seeing how vulnerable she is, she decides to stay to help. Though she’s been frightened by the house, and the girls, she realizes what they’re there, and decides to stay.
It’s also suggested that both Raleigh and Max had troubled childhoods, which perhaps contributed to them lacking a sense of purpose in the outside world. But unlike the young girls, they’re the ones that “made it” to adulthood, even if they couldn’t shake their trauma entirely. So they arrive at the house in a similar way as each other, and take the leadership role with the girls.
Between that, and my earlier theory about the girls sort of “clearing the way” for new girls to come, there seems to be a theme of “paying it forward,” in a way. Of these women and girls channeling their own trauma to help “resolve” the trauma of others (albeit in a gruesome way, but that’s the horror genre for you).