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 caterpillars 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.
Barbie's father Don plays an elaborate game to get Barbie to hand the egg over to him. Meanwhile, Big Jim learns that Julia has the egg and makes a deal with Don to get himself and Junior released from the Dome. And Pauline, Sam, and Lyle try to track down the red door and return to Chester's Mill to atone for their sins.
While Big Jim meets the wife that he believes died nine years ago, Barbie returns and tells Julia and Rebecca that there is a way to escape the Dome. When Junior learns that Sam has returned, he sets out to exact vengeance but a familiar figure tells him not to. Meanwhile, Joe and Norrie meet Hunter and track down the missing egg, only to discover that Big Jim warns it so that he can negotiate the release of his family.
The temperature inside the Dome drops, threatening everyone with a slow death by freezing. Meanwhile, Norrie and Joe begin to suspect that Hunter is still communicating with the outside world, and Lyle returns with a chilling prophecy of the future.
The Dome contracts, threatening to crush the residents of Chester's Mill. While Rebecca looks to science for an answer, Julia relies on faith. Meanwhile, Big Jim reconciles with Pauline and tries to help her gain a vision of a way to save Melanie, whose life is tied to the egg.
A potential exit from the Dome is revealed just as the walls begin closing in on those trapped in Chester's Mill.