Barbie is spared from execution when the new Dome phenomena generates an electromagnetic pulse that threatens to wipe out everyone. Meanwhile, Big Jim receives two mysterious visitors, and Julia receives help from a former EMT when a woman appears in the lake out of nowhere.
An infestation of butterflies threaten Chester Mill's food supply. While Barbie, Rebecca, and Big Jim deal with the threat to their crops, Joe and the others try to track down the killer.
Big Jim visits the town barber to get a haircut for his meeting with Rebecca., However, when red acid rain pours down, his plans are interrupted and Chester's Mill must find a way to survive what some believe is a Biblical plague. Meanwhile, Norrie and Joe go to the high school with the mystery girl and make two amazing discoveries.
Rebecca comes up with a means to eliminate 25% of Chester's Mill's population to save the other 75%. However, she wants Big Jim to carry out the plan. Meanwhile, Barbie, Joe, and Norrie learn where Melanie came from, while Junior comes to believe that Lyle murdered Angie.
After Big Jim and Rebecca's attempt to infect the townspeople, Julia has them put in jail and tries to organize a food drive. Meanwhile, Joe tries to help Melanie recover her memories, driving a wedge between him and Norrie.
While Barbie and Sam explore a mysterious tunnel beneath Chester's Mill, Big Jim tries to deal with a new crisis when a dust storm threatens the town's oxygen supply.
Barbie escape the Dome at last, but has to turn to an unlikely ally to find a way back in to reunite with Julia. Meanwhile, Joe confirms that the tunnel leads to the outside world, and Big Jim takes advantage of Barbie's disappearance to regain some of his influence in Chester's Mill.
Don offers to send a message for Barbie to Julia inside the Dome, but a mysterious hacker soon reveals that Don has an ulterior motive. Inside the Dome, new sheriff Big Jim suspects that someone in town is targeting him for revenge.
When Barbie is apprehended by a group of mysterious men, he is relentlessly interrogated about his connection to the Dome. Meanwhile, Big Jim makes a deal that could seal the fate of the residents of Chester's Mill forever.
Big Jim finally learns the shocking truth about what really happened to his wife, Pauline, when they are reunited. Meanwhile, climate change poses a new threat to Chester's Mill.
When temperatures begin to plunge, Sam and Rebecca spring into action to try to save the residents of Chester's Mill. Meanwhile, Barbie risks his own life in order to save Julia after a terrible accident.