Stranger Things Season 3 Episode 4
Chapter Four: The Sauna Test
The children join forces to find out what's happening to Billy.
They lock him in the sauna at the pool, and turn up the heat to confirm the Mind Flayer is taking up residence in his body.
When they do, he tries to kill everyone, including Max.
Eleven overpowers him, sending him through a wall.
Meanwhile, Nancy and Jonathan are fired from their jobs, prompting them to argue and subsequently split up.
Nancy's mother tells her to follow the story and to take it to a bigger market if she believes in it.
Nancy starts following it again, and finds herself in danger.
Dustin, Robin, and Steve recruit Erica into the group and ask her to climb through the vent to the loading bay.
She declines, but later agrees to if she gets a lifetime supply of ice cream. She calls the job child endangerement but agrees.
Billy has been stealing people from around town and making them part of the Mind Flayer's army, making things difficult for everyone.