As Hawkins nears the anniversary of Will Byers' return from the Upside Down his family works to press on, though Will is experiencing increasingly disturbing visions. Dustin and Lucas become obsessed with a new girl who has moved to town, and Hopper keeps a secret in the woods.
The kids head out to trick-or-treat on Halloween, with Lucas and Dustin inviting the new girl to join their party -- while Will continues to experience flashes of the Upside Down. Steve takes Nancy to a party in hope of getting her to move past Barb, and Eleven recalls the events that brought her to Hopper.
Dustin cares for the creature he found in his trash can, only to discover its true source. Eleven breaks Hopper's rules by leaving the cabin to find Will, while Hopper tries to convince the people at Hawkins Lab that their problem is not as contained as they believe. Joyce discovers something terrifying on the video tape from Halloween.
Joyce seeks help from Hopper after Will exhibits strange symptoms, giving them a new clue to the Upside Down's threat to Hawkins. Nancy and Jonathan get themselves taken into custody by Hawkins Lab, and Eleven is grounded following a powerful fight. Meanwhile, Dustin realizes the horrible truth about his new pet, Dart.
After entering the tunnels Hopper becomes trapped in the Upside Down, and Will's map is the only way to find him. Nancy and Jonathan seek out a new ally, while Dustin is left to deal with his problem on his own. Lucas shares the secret with Max, and Eleven meets her mother.
While Dustin recruits Steve for help with the rapidly growing Dart, Will's recovery from a major trauma brings new surprises. Meanwhile Lucas recruits Max for a mission, and Nancy and Jonathan confront their feelings.
Eleven travels to Chicago in search of the missing girl from her mother's memory, leading to a sort of family reunion with another of Hawkins' former experiments -- a her group of outcasts looking for revenge on the lab's former employees.
After a catastrophic incursion into Hawkins Lab the group find themselves trapped, as Hopper and Bob put their lives on the line. Elsewhere Steve, Dustin, Lucas, and Max track Dart, and Billy has a reckoning with his parents.
The group hatches a plan to return to Hawkins Lab, where Eleven believes she can close the gateway to the Upside Down -- while Joyce, Jonathan, and Nancy try to free Will from the control of the Mind Flayer.
