WARNING: The following article contains spoilers for Stranger Things Season 3, streaming now on Netflix.
The first season of Stranger Things certainly left fans wondering what else the Upside Down had to offer after we met the Demogorgon. Season 2 upped the ante in a big way by revealing its boss and the overall mastermind who wanted to infect Hawkins, Indiana, was none other than the Mind Flayer.
Thankfully, after it possessed young Will Byers (Noah Schnapp), his friends and family were able to exorcise it, with huge thanks going to Eleven (Millie Bobby Brown) for closing the rift to the otherdimensional realm and thus, cutting the Mind Flayer's link to the boy and the real world. However, Season 3 finds the Mind Flayer somehow influencing the town, with a plan that ends up being way more sinister and sneaky than first assumed.
BILLY THE BUILDER
With the Russians operating a secret lab under Starcourt Mall, Hawkins falls under the Mind Flayer's tricks once more as the Soviet scientists reopen the doorway to the Upside Down. We're not sure how long they've been operating in town, but it stands to reason it's after Eleven shut the door at the Hawkins lab in last season's finale. With the rift reopened, even partially, the Mind Flayer's able to possess rats, causing them to eat fertilizer and explode.
However, some clarity's gained on the overall plan when it attacks Billy after he crashes near Brimborn Steel Works and it possesses him. As it takes over his mind, we see Billy meeting a darker version of himself who urges him "to build" for the Mind Flayer.
We later find Billy kidnapping Heather (Francesca Reale), a fellow lifeguard at the Hawkins Community Pool, for the dark overlord to possess. At this point, it's clear Billy's task is to build an army... but for what?
FROM THE FLAYED... TO THE MEAT MONSTER
We discover later that more townsfolk are being infected and through the disease spreading, they're all falling under the Mind Flayer's control, even being called "The Flayed." When Nancy (Natalia Dyer) and Jonathan (Charlie Heaton) investigate Mrs. Driscoll (Peggy Miley) at the hospital after they also caught her eating fertilizer, they're attacked by colleagues from the Hawkins Post: Heather's dad, Tom (Michael Park) and his co-worker Bruce (Jake Busey).
The big twist in this fight comes when the men disintegrate into goo and form a Meat Monster that tries to eat the duo. Luckily, Eleven comes to their rescue, throwing the monster out the hospital, only to see it return to its liquified form and head to the sewers.
Soon after, we see the infected townsfolk at Brimborn -- Mrs. Driscoll included -- melting down (we're assuming the fertilizer and chemicals folks were ingesting were to liquify their insides and the rats were the Mind Flayer's prototype) and assimilating into the Meat Monster to become a much larger and unstoppable being to rip the town apart.
THE MIND FLAYER'S EGOTISTICAL ENDGAME
After a couple confrontations with Billy, and also due to Eleven probing his mind, the bigger picture comes into focus which is the Mind Flayer's using Billy, not to assemble an army for a physical brawl, but so that they can become the building blocks for this Meat Monster. This larger creature takes the shape of the Shadow Monster we saw in Season 2, all made from insides of Hawkins' finest, even boasting infected sleeper agents who leave the 4th of July celebrations to become one with the entity.
As for its goal, it wants to kill Eleven because it knows she's the only one who can shut the gate on it. Once she's out the picture, the Meat Monster can then "kill everyone," as Billy said, and basically turn this world into the Upside Down. With the Russians still trying to pry the gate open fully, well, it's all working out in the Mind Flayer's favor. However, its downfall comes due to the one human trait it exhibits though: narcissism.
That's because Billy has the opportunity to kill Eleven, especially after a smaller Meat Monster bit, infected and started depowering her as it demolished Hopper's cabin, but he surprisingly doesn't when he incapacitates her as she tries to hide in the finale, "The Battle at Starcourt Mall." That'd be too easy for the Mind Flayer. It wants Eleven to suffer in agony and it makes it clear that Billy has to bring the girl to it. It wants to devour her alive for itself and assimilate the girl into his body, which in its mind, is the ultimate revenge -- she'd become the very thing she hated and fought so hard against.
Created by the Duffer Brothers, Stranger Things stars Winona Ryder, David Harbour, Finn Wolfhard, Millie Bobby Brown, Gaten Matarazzo, Caleb McLaughlin, Noah Schnapp, Natalia Dyer, Charlie Heaton, Joe Keery, Priah Ferguson, Cary Elwes, Jake Busey and Maya Thurman-Hawke.