The only reason Billy knows that there's anything going on with what Max has dubbed "the Upside Down" is because, five days after coming home from the hospital, he coughs up black slime that, upon coming in contact with the oppressively hot Indiana summer, squeals and squirms and congeals in a manner reminiscent of the way the people he had lured into the General's grasp had after they'd melted into fleshy blobs.
So he coughs up the slime, or the Upside Down juice, or whatever the hell it is, and then he throws up at the sight of it and the memories it drags to the surface; when he goes to Max's room, tasting panic rising unwelcome like bile in his throat, she's gone, and her window is wide open.
The only bright side to this, he thinks, is that Neil is long gone, so even if Susan is furious at him for not paying enough attention to Max's whereabouts, she won't hit him. She might berate him, or kick him out, but she won't hit him because (and he knows this first-hand) it is harder to hit someone than it is to watch them get hit, and she couldn't even do that the one time she saw it happen.
He thinks that maybe his father had thought she was weak for that, but Billy cannot help but think that, in her own way, Susan is strong. She may not have loved Billy when she saw his father throwing him up against walls and introducing his hands to Billy's face and growling slurs at him, but she loved and loves Max enough to make sure that she won't get hurt.
Billy thinks that sometimes running away is the action that requires the most strength; he knows better than most, because his mother did it, and so did all the women after her. They ran away and saved themselves, and it was Billy who was too weak to do anything but stay and take it. Susan, like the others, has removed herself from the situation, but she didn't run. She forced Neil out, and she took her daughter with her; that's something that none of the others have done. They had the strength to take themselves, but not to take someone else.
Susan, Billy decides, is the strongest of them all. Max is lucky to have her as a mother.
Max is still missing, though, and Billy doesn't have a car or a clue how to find her. He'd kinda thought that, now that they were on better terms (and they've been on vaguely better terms since early November, because Billy may have wanted Max and Susan to hate him so that they'd stay, but even he couldn't tolerate just how far he'd gone; Max didn't need to be afraid of him to hate him, and so he'd apologized and done his best to stay out of her way), she wouldn't run off without telling him. But she's always done that, and Billy's not sure why he expected anything to change.
He may be clued in on the Upside Down stuff now, but he's still Max's asshole stepbrother whose redemption arc everyone has missed because he's had it while he left them alone, while he was possessed by the General, while he was unconscious and waking up and in the hospital and coming home to his father's departure. He's better now, he thinks, and he's been a work in progress for months, but the thing about making everyone hate him?
No one's around to see him changing in the shadows. He's a dick who gets better off screen, who goes from having a stepsister who hates him to a sister who loves him in the hours and days and months that nobody sees.
Billy thinks that it sucks a little – no, a lot – that he was so invested in this idea that he could only keep people around if they hated him; it only took three people to show him how wrong he was. It took Max and Susan – the mother and sister he thought hated him, who he now knows love him, staying; and it took Neil – the man he knew hated him, who was always there, who stayed through everything, looking over his shoulder, aggressively disapproving, heavy-handed and heavy-hearted, leaving.
And now Billy is left with the regret that comes with misunderstanding the world entirely, with the realization that he is alone because he has always lived in a different world than the people around him, with the knowledge that he is the only one who is aware that he's been changing since the night he wore a blood red shirt to a fistfight and tried to carve the fear of the world into a kid's bones…
Yeah, Billy's got a lotta regrets; he's got a head full of what-ifs and would've-should've-could've-beens, and he's only got himself to blame.
He wonders how many open doors he's slammed shut, wonders how many windows he's left open for escape, wonders if the hate he's spat in people's faces has done irreparable damage.
He wonders if the reason Max is missing has anything to do with the slime he coughed up and the Gate that he knows was open.
He moves towards the front door, intent on finding Max before she manages to get herself in too much trouble, when the world flickers around him, sliding from Hawkins' heat and blinding, mirage-inducing sun to a monotone world with a vine infestation that makes him feel sick.
The last time he saw this place, he had a body double standing in front of him. The last time he saw this place, he came back possessed, with the desire to hurt people and make the entire world rot engraved deeper than his bones.
This, he knows now, is the Upside Down, and the sight of it makes him scream, unthinking, for Max before the world shifts again.
He collapses to the floor, shivering, and for the first time since waking up, he wishes that he hadn't survived that night at Starcourt.
The bank keeps Susan late, so she isn't at dinner that night; when Max tells him where she was and what she was doing, there's no need for sneaking into bedrooms to exchange whispered words. They are able, for once, to have a conversation in their own home without fearing the repercussions. It's another thing to add to Billy's list of why it's good that his dad is gone. No more hate, no more beatings, no more fear, the ability to hold private conversations. He's sure that, in the coming days, the list will be so long that he'll forget what's on it.
It's a good thing that Neil's gone, but… Billy kinda misses his dad. He wonders what it says about him that the person he fears the most is someone he loves.
"Hopper's back," Max says through a mouthful of macaroni.
Billy frowns at her. "Swallow before speaking, Maxine. I didn't realize he was gone."
"There was a secret Russian base underneath Starcourt. That's where the Gate was. Hopper and Ms Byers closed it and Hop got stuck in the Upside Down. 'S where he's been the past twelve days. We've known he was down there since day… two, I think? He was calling Steve from down there, and then El got in contact with him, too. We had to open the Gate again, but he's out, and El was able to close it again."
Billy swallows. "I knew the Gate was open."
"You did? How?"
He shrugs. "I dunno. I coughed up some Upside Down slime, and then I just knew, and then it was like I was in the Upside Down for a second. 'S why I was in bed when you got home. Felt like shit afterwards."
She pales. "You were in the Upside Down? And there was slime?"
"That's what I just said."
Max shoves the last bite of her food in her mouth and heads over to the phone.
"What're you doing?"
"Letting the others know. It was happening with Will at the beginning of last year, around the time you and I first got here. It turned out he was possessed by the Mind Flayer, so we had to get it outta him and close the Gate."
"You think I might still be possessed?" Billy asks, suddenly feeling like he doesn't quite belong in his own skin.
Max regards him with a serious expression. "No. The Gate's closed. I think it was probably just a side effect of the Gate being opened again, but the others will want to know anyway."
He's not sure that he believes her, but Billy stays seated at the kitchen table and picks sullenly at his food while Max makes her phone calls. He hopes that the General – the Mind Flayer, whatever – is gone for good, and he has no illusions that he's the only one.
He expects that they'll be going to the Byerses' or Steve's or even Chief Hopper's, but instead the Hargrove-Mayfield household seems to stretch in an effort to fit two adults who aren't Susan or Neil, Jonathan Byers, Nancy Wheeler, Steve, Robin, four of Max's friends from school, some random dude with a very shiny head and a beard, and the girl who'd saved him from the General back in Starcourt.
She looks at him with huge eyes, sticks her hand out awkwardly, and reintroduces herself stiltedly: "I am Jane. It is good to see you are looking more like yourself."
Billy takes Jane's hand and shakes it carefully. She's tiny, and she looks fragile, but her hand is firm in his, and her expression is determined. "Thank you?" He's not entirely sure what she means by that, but it sounds like a good thing, at least. "I'm sorry," he adds, gesturing first at her neck and then at all of her; he's not really used to apologizing, and the way Hopper is hovering over Jane like a concerned parent tells him that saying something like: Sorry for dangling you in the air by your neck and trying to sacrifice you to the General would be a very bad idea.
"I'm sorry about… everything," he tells everyone else. He's apologized to Steve and Lucas for what happened in November, and to Ms Byers for breaking her stuff, and to everyone else for being a dick, but he feels like everything with the General needs apologies as well, and he's not quite sure how to give them, or who to give them to. It's a lot. He's done a lot of awful things to probably everyone in this entire town, but the truly terrible stuff seems to have been dumped on the strange group of people sitting in his living room.
It's Will – with his bowl cut, tired eyes, pale skin, and bones that seem even thinner than Jane's – that steps forward. His voice is soft when he tells Billy: "If you're apologizing for what you did under the General's control, don't. I know it feels like it's your fault now, but it isn't, and there was nothing you could've done." He looks around at everyone standing around them. "I would know. And if you ever want to talk about anything, I'm willing to listen. It helps. A lot."
Billy nods sharply, unable to speak around the tightness of his throat. Everyone else has already shrugged off his apology, or is offering him a smile, and he's not sure what to do with this easy forgiveness. He's used to no forgiveness and years-old grudges, to a mocking nod followed up with a fist or a foot. He's used to conditions, and they're giving him none.
"Sorry," Max says, her eyebrows drawn together in a way that emphasizes the way her mouth is turned down, "but who's the General?"
Will smiles sheepishly at her. "The Mind Flayer is the General. At least, that's the best description. They'd already chosen a name when I was possessed, and I kinda liked the ones my friends chose better than the one I knew, you know?"
Billy snorts. "You guys call the General the Mind Flayer? That's certainly an apt description."
"Yup," Dustin says, popping the p obnoxiously. Billy valiantly hides his irritation.
"Anyway," Hopper interjects, leaning forward. "We didn't come here for apologies or discussions about the General and the President or their alternative names. We came here to reassure you that the Gate is closed. Not only would El know if it weren't – and we all saw her close it – but so would Will and I. And I think that you would know, too. It's over, finally. We're all alive, and the Upside Down is closed."
Billy isn't the only one who breathes a sigh of relief that day and every day after.
The Upside Down is closed, and they're all alive.