‘Stranger Things’ Is Back. Here’s Where We Left Off in Season 3.

When the fourth season of “Stranger Things” kicks off on Netflix on Friday, nearly three years will have passed since the previous season was released, but only six months will have passed in the fictional town of Hawkins, Ind. Viewers may wonder why their favorite young characters are aging like the cast of “Grease,” but when you spend your childhood fleeing predatory humanoid creatures unleashed by an alternate dimension, you tend to grow up in a hurry.

The break between hauntings may not leave our heroes much time to catch their collective breath, but three years is an usually long gap between seasons, especially for a serialized show as dense with supernatural mythology, ensemble relationships and open-ended questions as “Stranger Things.” If you don’t have a spare 449 minutes to catch up with the third season in full, here’s what you need to remember.

Red dawn

Set in the summer of 1985, the third season took place at the height of Cold War tensions between the United States and the Soviet Union, before glasnost and Rocky Balboa started to thaw out their relationship. Looking for an edge beyond nuclear proliferation, the Soviets sneaked into Hawkins, where they deployed a giant laser beam to crack open the same gate to the Upside Down that Eleven (Millie Bobby Brown) and her friends labored so hard to seal up.

It was a little like the rationale Paul Reiser, who was introduced as Dr. Owens in the second season, used as a corporate villain in “Aliens”: If this powerful otherworldly force could be harnessed, it could be deployed as an unstoppable weapon of war. Who needs hydrogen bombs when you’ve got the Mind Flayer terrorizing the Heartland?

Happily for the citizens of Hawkins, the new Starcourt Mall has opened outside town, a pastel-colored consumer oasis with a Sam Goody, a Jazzercise place and a multiplex showing “Back to the Future.” Unfortunately, all the ma-and-pa businesses downtown are also starting to shutter, including the general store where Joyce Byers (Winona Ryder) clerks. And it turns out, those pesky Russians, through secret dealings with the corrupt mayor (Cary Elwes), have gobbled up the Starcourt and surrounding properties for their nefarious purposes.

Through intercepted and decoded Russian communiqués, Dustin (Gaten Matarazzo) and Steve (Joe Keery), along with Robin (Maya Hawke), Steve’s co-worker at the Scoops Ahoy ice cream parlor, uncovered the operation under the mall.

Re-animator

As it prepared to exert its psychic force on Hawkins once again, the Mind Flayer started possessing rats and humans, melting down their biomass and combining it to form a spider-like monster used to wreak havoc on earth. Some of the possessed, including the bad-boy lifeguard Billy Hargrove (Dacre Montgomery), returned to their lives as hallowed-out clones of their former selves, who became “active” at the malevolent entity’s discretion.

As interns at The Hawkins Post newspaper — the one downtown business that’s apparently still thriving — Nancy Wheeler (Natalia Dyer) and Jonathan Byers (Charlie Heaton) tried to get the scoop, but their deranged rodent story proved unfit for print.

Love will tear us apart

With the core characters moving deeper into adolescence, marathon sessions of Dungeons & Dragons were set aside for romantic intrigue — much to the annoyance of the dungeon master Will Byers (Noah Schnapp), who would rather have had life return to normal after his time in the Upside Down. Dustin returned from science camp raving about Suzie, who was reportedly “hotter than Phoebe Cates” but whose existence was questioned. (She exists. And loves the theme to “The Neverending Story.”)

Eleven and Mike Wheeler (Finn Wolfhard) were playing kissy-face all summer until Chief Jim Hopper (David Harbour), El’s adoptive father, put his foot down. That left Lucas (Caleb McLaughlin), who had his own on-again/off-again fling with Max (Sadie Sink), to give terrible advice based on what little he understood about girls. Steve used to understand plenty about girls, but he whiffed with Robin, who is into them herself.

It took most of the season for the kids to come together, so they have the Mind Flayer to thank for saving their fractured relationships. Nancy and Jonathan stayed on a low simmer as they investigated the rat-and-human possession story, and the will-they-or-won’t-they vibe between Hopper and Joyce continued, despite their obvious feelings for each other.

But they had to put their love on hold, too, after they kidnapped a Russian scientist and recruited Murray (Brett Gelman), the former reporter turned private eye, to translate info on how to infiltrate the Soviet operation and shut down the machine that has opened the transdimensional gate.

Fright night

The climatic episode turned the Starcourt Mall into a multilevel battleground over the Fourth of July, with some of the fireworks moved inside. In the mall atrium, the Mind Flayer, by way of the spider monster, squared off against the wildly overmatched kids.

A weakened El summoned every last drop of energy to beat back the monster. At the same time, her friends blasted away at it with a cache of stolen fireworks. For a moment, El manages to loosen Billy from the Mind Flayer’s psychic grip, and in a last-ditch moment of heroism, he sacrifices himself in order to save her.

As that was happening, Dustin and Erica (Priah Ferguson), Lucas’s little sister and an ice cream sample enthusiast, used Dustin’s radio to lead Hopper, Joyce and Murray through the tunnel system below the mall, where they posed as Russian agents to gain access to the giant laser. Although Hopper successfully battled a superagent, who was a dead ringer for Robert Patrick’s T-1000 in “Terminator 2,” he was also vaporized in the course of destroying the machine. Or so it seemed …

Every time you go away

Earlier in the season, Hopper had vowed to make Joyce feel like Hawkins was a safe place for her to call home. With his presumed death putting an end to that promise, Joyce finally decided to move out of the cursed town that had tormented her family so relentlessly. She and her boys moved to California, joined by El, who had lost both her adoptive father and her powers while fighting the Mind Flayer.

In a postscript set in a Russian military complex, the guards sought out a prisoner to feed to a Demogorgon, somehow captured from the Upside Down and kept in an enclosure, like a velociraptor in “Jurassic Park.” The guards were told not to pick “the American” for the Demogorgon’s lunch, which left fans with the obvious question: Who is the American? Is it Hopper?

A trailer from early 2020 confirmed that it was, indeed, Hopper. Based on Netflix’s release last week of the first scene (a video since taken down), he and El will have a lot of catching up to do if they manage to reunite.