S3E3 · Chapter Three: The Case of the Missing Lifeguard

Complete story — includes the ending

While having a sleepover with Max, Eleven jokingly uses her powers to spy on Mike, Lucas, and Will. She later spies on a suspicious and strange Billy, who senses her presence. She and Max discover that Heather has gone missing. Nancy and Jonathan find reports of missing fertilizer and strange behavior among rats across town, and they discover Mrs. Driscoll herself eating fertilizer in her home. After his attempts to play Dungeons and Dragons like old times, Will fights with Mike and Lucas as they are upset over Eleven and Max. He retreats to Castle Byers, and upon recalling his childhood memories with his friends, he destroys it. Robin decodes the Russian communication, which leads to a shipment arriving at Starcourt that night precisely at 21:45; she, Steve, and Dustin spot armed Russian soldiers handling the delivery. Joyce forces Hopper to go to the abandoned Hawkins Lab to answer queries they have about the falling magnets; when they investigate, Hopper is attacked by Grigori, a Russian soldier. Eleven and Max track down Billy and Heather at the home of Heather's parents. After they leave, Billy and Heather subdue her parents, which Will senses while Mike and Lucas find him, and he reveals to them that the Mind Flayer has returned.