The fourth episode of Stranger Things’ third season, Chapter Four: The Sauna Test, starts with the upcoming Fourth of July celebration in the town of Hawkins. The team is still trying to locate the Mind Flayer’s lair and is beginning to devise strategies for stopping the threat that comes from the Upside Down.

As the team is devising their plan, they discover that the Mind Flayer had previously attempted to infiltrate the small town unnoticed by possessing hosts. They decide to test Billy to determine if he’s been possessed. Nancy and Jonathan also decide to investigate further into the scandal involving the Hawkins Post and the strange illness caused by the lab’s radiation.

Meanwhile, the Russians are working on the gate to the Upside Down underneath the Starcourt Mall, led by the Terminator-like assassin Grigori. Hopper and Joyce decide to look for a way into the underground bunker to destroy the gate, while also worried about the growing chemistry between Eleven and Mike.

As the episode progresses, Billy, now apparently under the influence of the Mind Flayer, begins to spread the Flayer’s possession to other townspeople. Will starts sensing disturbances from the Upside Down, and the group devises a plan to use high temperature to force out the Mind Flayer from their host.

They lure Billy to the local pool and execute the plan, using a sauna to raise the temperature and force the Mind Flayer out. The plan works but not before stressing out Eleven, who struggles to use her telekinetic powers due to her stress. The Mind Flayer reveals itself, explaining that it intends to destroy all humanity by opening the gate and creating an army of possessed humans.

During the aftermath, Nancy and Jonathan are attacked by a possessed Mrs. Driscoll and are forced to flee, while Hopper and Joyce discover the secret entrance to the Russian base under the mall. The episode ends with both groups still struggling to overcome the obstacles in their way, setting the stage for the climactic episodes yet to come.