The main plot of Season 3 is that the crew believes that Russians are attempting to interfere with the Upside Down and do a great deal of harm to the United States through the power in the alternate dimension. Seems pretty absurd, right? Well, not really.
Dustin works with Steve, Maya, and Lucas' younger sister, Erica (Priah Ferguson), to break into the secret area they're using for storage in the Starcourt Mall and figure out what's really going on.
On the flip side, Hopper, Joyce, and their new Russian buddy, Dr. Alexei (Alec Utgoff), seek out more information about the Russians by paying a visit to Murray Bauman (Brett Gelman) to help them both translate for Alexei and investigate the situation. With Bauman's help, they learn that the Russians are trying to break through the barrier to the Upside Down in order to harness and weaponize the energy and creatures from that dimension. So basically, Dustin was right.
The crew comprised of Mike, Eleven, Will, Lucas, and Max realize that even though the Mind Flayer has left Will's body, it hasn't left the planet just yet.
Suspecting that the Mind Flayer has taken over the body of Billy Hargrove (Dacre Montgomery), the older step-brother of Max, the crew puts him to a test in order to confirm their suspicions. As the Mind Flayer hates the heat, they turn up the degrees as Billy takes a steam in a sauna, quickly confirming that the Mind Flayer has overtaken him.
They head over to Hopper's cabin to take cover from the Mind Flayer, which then proceeds to destroy the cabin little by little as it tries to kill each member of the group. At this point, the Mind Flayer is growing larger and larger as it takes over the Flayed members of the town. While they're able to ultimately hold their ground at the cabin, Eleven gets injured in the process.
With a good bulk of the season taking place at the Starcourt Mall, it was only right for the largest battle we've seen yet to go down in that very place. Following the mess at the cabin, we change course over to the mall where an even larger version of the Mind Flayer has arrived. A pretty angry monster, it kills Billy right in front of Max, who's upset and disturbed by this despite their complicated relationship. As always, it genuinely seems like the crew is done for when Joyce switches off the portal from the Russian Operation room, thus killing the monster.