Spoilers for Stranger Things [1] season three ahead, FYI!

Both seasons of Stranger Things [2] thus far have roughly followed the same formula: government conspiracy + creepy monsters + resourceful kids armed with slingshots, hormones, and telekinesis = binge-able Netflix gold. While season three is much of the same, the show's creators, Matt and Ross Duffer, have also introduced a far gorier, complex mystery than usual, which all begins with one key question — what the hell is wrong with Billy Hargrove [3]?

As a reminder, Billy is introduced in season two as the metal-loving, mullet-rocking, physically and verbally abusive older brother of Max (Sadie Sink), who is forced to move to Hawkins so her family can have a fresh start from their lives in California. He's a bully through and through and doesn't inspire much sympathy. At the end of the second season, we discover that he's learned his behaviour from his father, who's just as physically and verbally violent as he is.

In season three, which opens in the heat of Summer in Hawkins, the character is once again played with searing, swaggering intensity by Dacre Montgomery. He's a lifeguard at the public pool, where he flirts daily with Mike's mom, Karen Wheeler (Cara Buono), and goes so far as to arrange a "date" with her at the local Motel 6. Karen backs out at the last second, which is good since Billy doesn't show either — he crashes his car on the way there while passing an abandoned steel factory in town. That's when things get real.

How Does Billy Come in Contact With the Mind Flayer?

When Billy gets out of the car to inspect the damage to his windshield, he's lured inside the warehouse and dragged screaming down to the basement where what remains of the Mind Flayer monster [4] is lurking. It "infects" him the way it infects Will in season two: it clamps a seriously disgusting tentacle over Billy's mouth that allows the monster to control his every move from then on (the way a demon possessing someone in a horror movie does). Like Will, Billy retains some sentience and is able to escape the warehouse and run outside, but there he encounters a vision of the Upside Down. The Mind Flayer, who appears as Billy himself, approaches and says, "I want you to build, I want you to build what you see."

What Does the Mind Flayer Want Billy to Do?

So, what does that mean? Well, essentially the Mind Flayer — who everyone starts referring to as "he" throughout the season — wants Billy to bring him a bunch of human sacrifices that the monster can also infect; it seems he wants to build an army of possessed Hawkins-ites so that he'll eventually be powerful enough to defeat Eleven (Millie Bobby Brown). Billy just happened to be the first person he came across.

How Did the Mind Flayer Even Get Into Our World If Eleven Closed the Gate?

Remember in the end of season two when Joyce, Jonathan, and Nancy bring Will to Hopper's cabin and make the room superhot in an effort to drive the Mind Flayer from Will's body? And remember how they succeed, sending a plume of black smoke shooting out of him and up into the night sky? Well, apparently at that same exact moment, Eleven had closed the gate. So, Will's piece of the Mind Flayer had nowhere to go and was left to fester in our world until it mustered up enough strength (read: consuming enough poisoned rats) to move on to new human hosts.

What Happens to Billy After He's Possessed?

It doesn't take long for Billy to put his glistening, shirtless [5], six-packed days at the pool behind him. Instead, thanks to the Mind Flayer liking it "cold," he appears to be allergic to the sun and heat, spending his time under an umbrella with most of his skin covered up. Underneath his long sleeves are gross dark veins and sores on his body, and his anger management problems increase tenfold.

Billy's first victim is his coworker, Heather, who's a popular lifeguard at the pool and the daughter of the Hawkins Post's editor-in-chief (aka Nancy and Jonathan's new boss). He kidnaps her and drags her to the warehouse, where the Mind Flayer infects her, too. Then they get her parents. And a neighbour. And so on. See where I'm going with this?

So, Does Billy Ever Turn Into a Monster, Too?

Actually no, he doesn't! Colour me surprised. By the end of the season, Billy and Heather have amassed a pretty significant group of people infected by the Mind Flayer's influence. Like the infected rats, though, their fates are pretty gruesome — when the monster decides he's ready to fight Eleven, he has his subjects line up in the warehouse in front of him, and they proceed to explode into big red puddles of writhing goo. One by one, the puddles combine to form a giant, slimy, physical version of the Mind Flayer that we've seen only in the form of black smoke thus far.

Lucky for Billy, he escapes this particular fate. Instead of dissolving into goo to be sucked up by the Mind Flayer, he teams up with the monster at Starcourt Mall to take down Eleven, Mike, Nancy, Jonathan, and the rest of the crew that's gathered there. But thanks to some poking around in his brain, Eleven is able to emotionally connect with Billy in a pivotal moment, and his true self breaks free of the Mind Flayer's hold long enough to save her. The monster repays Billy's act of bravery by . . . immediately impaling him six ways to Sunday.

Billy doesn't get a chance to have a full-blown redemption arc or time to heal his relationship with his sister, but at least he makes his last seconds count. As he bleeds out on the floor of the mall, his sister Max rushes over, and Billy utters one simple but meaningful word to her: "Sorry."