Cold open: Just because you've encountered the Mind Flayer walking among us doesn't mean you can't take time out for dental hygiene. So good on Eleven (Millie Bobby Brown) for brushing her teeth and flashing back to the things we just saw happen in Episode 3 involving Billy (Dacre Montgomery), Heather (Francesca Reale), and the Mind Flayer. (Stranger Things 3 is definitely built like "one long movie," but scenes like this -- expositional recaps within episodes -- are decidedly old-school television work.) Done brushing, Eleven heads back to the bedroom to hang out with Max (Sadie Sink), who's convinced Billy and Heather are totally fine and not at all possessed by a demon hell beast. Good instincts! Eleven isn't so sure, but she takes a break from the Upside Down to learn a little bit about Wonder Woman. "This is why you can't just hang out with Mike all the time," Max tells her, opening a comic book. Amen, Max.
Meanwhile, that same night: Nancy (Natalia Dyer) and Jonathan (Charlie Heaton) watch as Mrs. Driscoll is wheeled into an ambulance screaming like ... well, Jack from Lost: I have to go back, she shrieks. As the elderly woman is sped to the local hospital -- ingesting fertilizer is probably bad? -- she reaches out in desperation to the abandoned steel mill where Billy was first possessed.
Inside the structure, Heather's parents are tied up and scared -- and her father, Tom the Hawkins Post editor-in-chief (Michael Park), pleads with his daughter to stop what's happening. "There is no stopping it, daddy," not-Heather says as Billy arrives and the Natural Born Killers walk away, leaving the Mind Flayer to his latest prey. And for the first time, we see in full bloom what happens when it attacks: tentacles attach to faces, Alien-style. As always, Netflix subtitles light the way: [muffled grunts] [creature snarling]. Hit the credits!
Hopper and Joyce: Following his run-in with Darth Drago, Hopper (David Harbour) wakes up naked with Joyce (Winona Ryder) tending to his injuries. It's not as sexy as that sounds: Hopper's clothes were soaked and he promptly throws up upon becoming conscious. Better luck next time on the Jopper front. As it turns out, Joyce was able to get a few letters and numbers off the Russian henchman's license plate, but Hopper is unconvinced the bad guy will have registered a car under a legal name. But it wasn't a car, Joyce says. Lightbulb time for Hopper! Who did he see at the mayor's office carrying a motorcycle helmet? (Duh, Darth Drago!)
At the mayor's office, Hopper goes full Martin Riggs, breaking Mayor Kline's (Cary Elwes) nose and forcing him to explain why Darth Drago was even at City Hall. As it turns out, he works for Starcourt and the company is looking to expand its reach to properties in East Hawkins -- specifically a handful of farms in the area. Joyce and Hopper head out to investigate -- and we'll catch up with them in the next episode! (Sorry, folks, not much Hopper and Joyce in this one!)
Dustin and Steve and Robin: Having found the secret storage room at Starcourt Mall where the Russians are hiding stuff, Dustin (Gaten Matarazzo), Steve (Joe Keery), and Robin (Maya Hawke) try to figure out how to get inside while avoiding the heavily armed guards who stand watch. Once again, it's Robin who saves the day: She heads to the county recorder's office to get the Starcourt Mall blueprints and finds that the mall's air ducts can be used to climb into the room. All they need is someone small enough to fit through the vents. Good thing Lucas' sister, Erica (Priah Ferguson), has been hanging around the ice cream shop all season! All it takes to convince her is lots and lots of ice cream.
With the mall closed, Erica climbs through the ventilation system like John McClane, which is a reference the kids from Stranger Things wouldn't actually get until three years from the events of Season 3 (but that the Duffer Brothers certainly know well). She reaches the storage room with ease and Dustin, Steve, and Robin soon find themselves inside. So, what's in the boxes? Cylinders of a glowing green substance. As our heroes reckon with the discovery of the show's latest MacGuffin, something strange happens: The room is no room, it's an elevator. And, like that, it starts dropping down a hidden shaft at high speed.
Nancy and Jonathan: With Mrs. Driscoll hospitalized, Nancy and Jonathan are called into Tom's office -- yes, the same Tom we saw being flayed by the Mind Flayer the start of this episode. He admonishes Nancy and Jonathan and fires the amateur reporters for their insubordination -- all while gaslighting them into thinking everything that happened is not a real concern.
Later, as the pair drive away from the confrontation, it's time for some relationship drama. Nancy calls bull on Tom's comments, but Jonathan isn't having it; he thinks she's privileged and that the world sucks and that the sexist men who belittled her also gave them jobs, so all's fair?
Back at her house, Nancy has a heart-to-heart with her mom, Mrs. Wheeler (Cara Buono), who takes Jonathan's "the real world sucks" comment and turns it on its ear. "Most people stop trying. But you're not like that. You're a fighter. You always have been." Invigorated, Nancy is ready to go once more unto the breach and heads to the hospital to look in on Mrs. Driscoll -- who's having a Mind Flayer meltdown because of what's happening in another storyline...
Mike and Eleven and Lucas and Max and Will ... and Billy: The morning after Will (Noah Schnapp) felt the Mind Flayer return, Mike (Finn Wolfhard) and Lucas (Caleb McLaughlin) are desperate to get back in touch with Eleven and Max and Dustin. The boys succeed in convincing Eleven and Max to show up (Dustin is still MIA, dealing with the Russians lol what a parenthetical!) and Will explains that his flayed feelings started to return the night of the blackout. My sneaky favorite thing in serial storytelling: when people communicate with each other!
Anyway, Will recaps what happened to him during Stranger Things 2 and the kids are able to piece together that the Mind Flayer is back and looking for a new host.
At the town pool, the five friends spy on Billy, who's covered head to toe in clothes to avoid the sun. Max isn't convinced he's gone bad (the power of denial is strong), but the others are less sure. Either way: How can they activate the Mind Flayer inside Billy to prove his possession once and for all? Look to the episode title, "The Sauna Test." Mike suggests waiting until the pool closes and using a dummy (Ferris Bueller would be so proud) to draw Billy into the sauna. Once inside, they'd lock the door and turn up the heat.
Good plan, but Mike's self-satisfaction is knocked down a peg when he chats with Eleven, who calls him out for saying girls are "a different species." Mike realizes that means Eleven spied on the boys -- and definitely saw Lucas rip a giant fart, but we won't talk about that right now -- and whines about the rules. "I make my own rules," Eleven says in another mic-drop quote. Dunk on Mike all day, El!
Nightfall has hit the pool and after Billy takes another gratuitous shower, the kids put their plan in action and lock him inside the sauna. Billy goes full Mark Wahlberg in Fear as he struggles with the rising heat before having an existential breakdown to Max. "It's not my fault," he cries. "I've done things, Max. Really bad things. I didn't mean to. He made me do it." Is that the sad, scared Billy breaking through the Mind Flayer's grasp or just a clever ruse by the Mind Flayer to get Max to let her guard down? We don't know and don't have a lot of time to consider it anyway: Billy breaks out of the sauna in spectacular fashion, the Mind Flayer overtaking his body and giving him super strength. (Scary, sad part: Billy verbally abuses Max, saying, among other things, that he'll "f--ing gut" her.)
What happens next is the most thrilling part of the season so far: Eleven and Billy have a knock-down, drag-out fight -- which ends quickly, with Billy getting the upper hand and grabbing Eleven by the throat. Things look bleak until Mike comes to the rescue. He smashes Billy with a bat ("Go to hell, you piece of sh--") and momentarily slows down his mayhem just long enough for Eleven to recover and literally toss Billy through a brick wall. The pleasure is only momentary: The Billy Flayer is fine and runs away for another day.
Back at the steel mill, Heather is tending to Billy's wounds. He knows Eleven could have killed him. "But not us," Heather says as the camera pulls back to reveal a room full of possessed Hawkins residents, all of whom look like Westworld hosts. Spooky!
Movie of the episode: "Come out to the coast, we'll get together, have a few laughs." Die Hard is streaming on numerous digital platforms, including Vudu.
Quote of the episode: Leave it to Erica to provide another winner as Dustin tries to convince her to help them stop Russia from invading the United States. "You can't spell America without Erica." My country 'tis of thee...
TV Guide Episode Rating: 4 out of 5
Stranger Things is streaming now on Netflix
Up next: Episode 5, The Flayed