It's summer in Hawkins, and the town's new mall is the place to be. While Hopper tries to figure out how to handle two hormonal teenagers, Dustin returns home from camp -- all unaware that a sinister new threat is looming.

After Hopper manages to drive a wedge between Eleven and Mike, Max takes her for a day at the mall. While Nancy and Jonathan use their newspaper connections to investigate a strange rat problem, Dustin turns to Steve for help translating a Russian code he has intercepted -- and Billy must come to terms with his close encounter.

After a disturbing astral projection Eleven and Max look for Billy, and discover that a fellow lifeguard is missing. Steve and Dustin go in search of evil Russians, while Will grows frustrated with his friends' new priorities.

Suspicious that Billy may be host to the Mind Flayer, Will and the others hatch a plan to trap him. Hopper goes to the Mayor for answers, and Robin, Steve, and Dustin look for help infiltrating the Russians' secret operation.

Hopper and Joyce discover a secret Russian facility and barely make it out alive -- along with one of the scientists, who doesn't speak English. Meanwhile Dustin, Steve, Robin, and Eric find themselves in the heart of enemy territory, and Nancy and Jonathan return to the hospital in the hope of tracking the Mind Flayer back to its source.

Nancy and Jonathan battle the Mind Flayer inside the hospital, while Hopper tries to convince his prisoner to cooperate. Robin and Steve are interrogated inside the secret Russian base, and Eleven puts herself on the line to locate Billy and the Mind Flayer.

The kids are targeted by the Mind Flayer, who has discovered their location and wants to take El off the board. After escaping the Starcourt compound Dustin and Erica take their friends to lay low, and Joyce and Hopper get more than they bargained for at the mayor's Fun Fair.

With Eleven out of commission following her fight with the Mind Flayer, Hopper leads a mission inside the Russian facility to shut down their deadly project -- while the kids find themselves under siege inside the mall.