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.
When Barbie and Sam set out to investigate a mysterious tunnel, a cave-in severs their path back to Chester's Mill. Meanwhile, Julia and Big Jim face off in a struggle for power as a dust storm rages in the town.
When Barbie descends into the unknown abyss in the mysterious tunnel to look for Sam, he discovers a world that is familiar but filled with unanswered questions.
When Barbie enlists his father's help to reach out to Julia, he realizes that Don may know more about the Dome than he is letting on. Meanwhile, Big Jim appoints himself sheriff of Chester's Mill.