The team finds out that there really is a Russian science bunker under the mall (and yes, that's a sentence you just read). Turns out, the Russians made a deal with Mayor Kline to "develop" Hawkins in exchange for using the town's proximity to the Upside Down to extract some sort of toxic substance from the dark dimension. There's a Russian super-soldier, Grigori, on a collision course with Hopper and Joyce's own investigation, and Nancy and Jonathan fight their possessed ex-bosses, who then dissolve into fleshy goop and meld together into a new Mind Flayer monster.
Turns out, the Mind Flayer has been busy controlling a whole bunch of humans to do its bidding, and the whole crew has to team up to stop its spider-monster host, which results in El being seriously weakened and her powers disappearing. The team realises they have to close off the Gate again, cutting off the spider monster from its "brain" in the Upside Down. After several setbacks, they're finally able to do it — but at a huge cost. Joyce is forced to blow up the Gate opener with Hopper in the room, seemingly killing him.
Three months later, in late Summer/early Fall of 1985 the Byers family — now including the recently re-orphaned El — packs up to move out of Hawkins. And in Kamchatka, Russia, a pair of guards choose a prisoner to be fed to a new Demogorgon — but they specify "not the American."