Joyce Byers (Winona Ryder) — the mother of Will and Jonathan — and former investigative journalist Murray Bauman (Brett Gelman) spent much of season four tracking down Jim Hopper (David Harbour) after he was taken captive by Russians at the conclusion of season three.
By episode seven, they've finally tracked him down to a prison in Kamchatka, Russia. Unfortunately, he's being forced to fight the Demogorgon, a monster from the Upside Down that first emerged in the show's first season, along with Dmitri (Tom Wlaschiha), the Russian guard who attempted to help him escape.
With the help of Joyce and Murray, who open the doors to the arena where Hopper and Dmitri are fending off the creature, they survive the fight. Hopper finally shares an emotional reunion with Joyce.
At the end of the episode, all four characters — Joyce, Murray, Hopper, and Dmitri — are presumably still free agents within the prison.