The popular new Starcourt Mall has caused many of Hawkins' other stores to close. Jim Hopper intervenes in Mike Wheeler's relationship with his adopted daughter Eleven, and Eleven becomes friends with Max Mayfield. Still recovering from Bob Newby's death, Joyce Byers considers moving out of Hawkins with her children. Magnetic disruptions lead her to believe that the Upside Down has returned, though, and she enlists Hopper's help in uncovering the cause. While Mike and Lucas Sinclair attempt to repair Mike's relationship with Eleven, Will Byers begins experiencing premonitions from the Upside Down even though Eleven closed the original gate, believing the Mind Flayer, a many-armed shadow creature from the Upside-Down which previously possessed him, is still alive in Hawkins. As Nancy Wheeler and Jonathan Byers investigate the effects of the Mind Flayer's influence, Dustin Henderson, Steve Harrington, newcomer Robin Buckley, and Erica Sinclair begin investigating a potential Soviet infiltration of Hawkins. Meanwhile, Max's stepbrother Billy Hargrove is taken over by the Mind Flayer, who possesses other citizens of Hawkins to fulfil its plan.
Summer brings new jobs and budding romance. But the mood shifts when Dustin's radio picks up a Russian broadcast, and Will senses something is wrong.
Nancy and Jonathan follow a lead, Steve and Robin sign on to a secret mission, and Max and Eleven go shopping. A rattled Billy has troubling visions.
With El and Max looking for Billy, Will declares a day without girls. Steve and Dustin go on a stakeout, and Joyce and Hopper return to Hawkins Lab.
A code red brings the gang back together to face a frighteningly familiar evil. Karen urges Nancy to keep digging, and Robin finds a useful map.
Strange surprises lurk inside an old farmhouse and deep beneath the Starcourt Mall. Meanwhile, the Mind Flayer is gathering strength.
Dr. Alexei reveals what the Russians have been building, and Eleven sees where Billy has been. Dustin and Erica stage a daring rescue.
With time running out - and an assassin close behind - Hopper's crew races back to Hawkins, where El and the kids are preparing for war.
Terror reigns in the food court when the Mind Flayer comes to collect. But down below, in the dark, the future of the world is at stake.