S3E4 · Chapter Four: The Sauna Test

Complete story — includes the ending

Billy and Heather bring her parents to the mill, where they are possessed by the Mind Flayer, now a physical creature composed of flesh from the rats. Hopper recalls Grigori meeting with town mayor Larry Kline; he and Joyce force Kline to reveal that Starcourt Mall is a Russian front for buying abandoned properties across Hawkins. At Starcourt, Dustin, Steve, and Robin, alongside Lucas' sister Erica, discover that the mall's loading dock is an elevator, which traps them beneath the mall. Nancy and Jonathan are fired by their boss Tom, Heather's father, for harassing Mrs. Driscoll; Nancy visits Mrs. Driscoll at the hospital out of suspicion and watches her become possessed by the Mind Flayer. Will reveals his connection with the Mind Flayer to Eleven, Mike, Lucas, and Max, speculating that it has possessed Billy. They devise a plan to trap Billy in the pool's sauna to confirm he is possessed, but Billy escapes and nearly kills them until Eleven subdues him. Billy returns to the mill, where dozens of townspeople have since been possessed by the Mind Flayer.