Five months have passed since the ending of Stranger Things 2. It is now late May, 1987. It was brought to the viewers attention at the end of Season 2 that Eleven had not destroyed the upside-down, and that it was still cultivating under the town of Hawkins. Dr. Martin Brenner is still alive, and is running a disclosed laboratory in the town which Kali/Eight lives in. It comes to Dr. Brenner's attention that the upside-down and the shadow monster is still lurking underneath Hawkins. Dr. Brenner insists on opening back up the upside-down so he and his 'lab rats' can identify what exactly the shadow monster is after. In order to do so, he must first get Eleven back under his commands. Meanwhile in Hawkins, the gang (Mike, Eleven, Dustin, Will, Lucas, and Max) have had a pretty average summer, and school is about to begin. Since the upside-down gate has been closed, no Mind Flayers or Demogorgons have been able to attack any of the people of Hawkins. Dr. Brenner decides the best way to capture Eleven is to distract her using somebody that her mind can not resist. That somebody is her mother, Terry Ives. Dr. Brenner and his 'lab rats' attack Kali/Eight's gang. They kill all of her friends, and kidnap her. Dr. Brenner punishes Kali from escaping and tortures her. Kali is forced to follow the commands of Dr. Brenner if she wishes to live. He uses her powers to create an image of Terry Ives, Eleven's mother. Kali arrives in Hawkins and forces Eleven to visualize her mother. Eleven falls in a trance and follows her "mother" outside of Jim's home. Dr. Brenner kidnaps Eleven. School is beginning in Hawkins and no one has heard from or seen Eleven. Back at Jim's home, he is beginning to wonder why Eleven has not eaten her Eggos, and goes to her room to wake up her. She is gone. Mike, Eleven, Dustin, Will, Lucas, Max, Joyce, Jim, Nancy, Jonathan, Steve, and Dr. Sam Owens must work together to find Eleven and stop her from reopening the upside-down. The gang is too late, and Will begins to suffer from more crazy symptoms. Will's sickness is different from the last time, and he seems to be dying. The Mind Flayer purposely left a small part of itself in Will so that if it ever had a chance to return, that it could instantly begin to feast off of Will as its host. Instead of attacking Will's mind, the Mind Flayer is trying to escape from Will and become a figure of present-day Earth. The gang must save Will, find Eleven, stop Dr. Brenner, and re-close the upside-down. Meanwhile, Nancy, Jonathan, and Steve continue their love circle, and Billy is found out to be another lost child from the Hawkins laboratory (No. 006/Six). Two new characters, Clint Esteen and Martha Childs, are introduced. They are ghost hunters and have information regarding Dr. Brenner and his laboratory, and agree to help the gang end the upside-down evil for good.
