As we catch back up with the good (and occasionally bad) citizens of Chester's Mill, the town is suddenly under threat from the dome -- in the form of flashing lights and strange, loud groans. Just after Julia (Rachelle Lefevre) throws the dome egg into the lake, an unfamiliar man helps her save a mysterious young girl from drowning; turns out he's Sam (Eddie Cahill), a former EMT who lost his job due to alcoholism. He seems to recognize the girl, who eventually disappears from his cabin and begins silently wandering through the town.
Meanwhile, the light and noise begin just as Big Jim (Dean Norris) and Junior (Alexander Koch) are about to hang Barbie (Mike Vogel) for his supposed crimes. Junior, still mad at dear old Dad, can't get himself to pull the lever. Suddenly, almost everyone around them faints. Linda (Natalie Martinez) takes matters into her own hands, forcing Big Jim to let Barbie go so they can all go and check out the dome for answers.
A super-strong magnetic pull appears to be coming directly from the dome during each loud groan; Joe (Colin Ford), Norrie (Mackenzie Lintz), and Angie (Britt Robertson) watch as the church's bell is ripped from its steeple, smashing hard into the dome's wall. Over at the dome's edge, the magnetic pull is way too strong; it tugs at Barbie's metal handcuffs -- and though Linda manages to get them off in time, Big Jim's car goes barreling toward her. Sadly, she can't be saved. Barbie blames himself, and Big Jim blames Junior for not killing Barbie when they had the chance. Junior fires back, making Big Jim finally admit that he murdered Dodee ... and doesn't regret it one bit.
Later, Barbie meets Rebecca (Karla Crome), the high school's science teacher, who tells him her theory: The groans are magnetic pulses -- and if they don't stop them, everyone in town could die. The pair helps to save Joe, Angie, Norrie, and an unconscious Carolyn (Aisha Hinds) by taking them to the school -- now the safest place in Chester's Mill, as it's as far away from the dome's pull as possible -- and they hatch a plan to put together their own huge magnet that will hopefully counteract the dome.
Big Jim winds up trapped in his own bunker -- and he's confronted by a vision of the dome's still-spunky version of Dodee (Jolene Purdy). She explains that this whole situation really is his fault, and that he better decide fast if he's willing to sacrifice himself for the town ... especially his son.
Another groan causes Junior and Angie to faint as well; while dreaming, Junior sees images of a strange city called Zenith, and is totally shocked to sees his very own mother (Sherry Stringfield) standing before him. Later, back in reality, we actually see his mom, alive and well; she's in a dark room, listening to a report about the dome while painting -- all in the shadow of a similar cityscape.
Barbie and Julia happily reunite just as Big Jim becomes further convinced -- this time by a vision of the dome's version of recently departed Linda -- that he must kill himself to stop the dome's threat. At first, Julia believes him ... but she soon realizes the dome wants them to stop the killing, and she saves Big Jim's life. The dome above them clears, the groans stop, and the townspeople thankfully come back to life.
Sam arrives to town looking for the mysterious girl, and it's discovered he's actually Big Jim's brother-in-law (a fact Junior finds more than coincidental, after the vision of his mother). Later that night, Angie follows the girl to the school. When Angie calls to her down a dark hallway, the girl suddenly snaps out of her trance-like state -- but runs away. Just then, Angie is violently attacked. Her bloody handprint is all that remains on a school locker...