Big Jim (Dean Norris) is in shock -- is Pauline (Sherry Stringfield) another Dome illusion? She explains how she faked her death (with the help of Lyle) nine years earlier; she believed if she left, she'd "take" the Dome with her -- saving everyone, especially Junior (Alexander Koch). Meanwhile, Rebecca (Karla Crome) and Julia (Rachelle Lefevre) notice the cold weather, and how the leaves have started to turn. Suddenly, Barbie (Mike Vogel) appears, and Julia runs into his arms. Norrie (Mackenzie Lintz) and Joe (Colin Ford) realize Melanie (Grace Victoria Cox) hasn't been home for hours, and that the egg is missing too. Down in the bomb shelter, Melanie wakes up to find Junior watching the egg. As she leaves, the Dome's version of Angie materializes; she tells Junior he'll want to follow his heart -- but he shouldn't. Melanie overhears and comes back, wondering if Junior was speaking to her. As she leaves again, Junior's vision of Angie disappears.
Barbie explains to Rebecca and Julia exactly how he found himself in Zenith -- and how he, Pauline, Sam (Eddie Cahill), and Hunter (Max Ehrich) made it back under the Dome. But what happened to Lyle? The important thing is that they all now have a way out, and (a somewhat skeptical) Julia goes to call a town meeting. Sam finds Melanie and offers an apology; Melanie counters, believing the Dome might've never come if they'd all helped her protect the egg in the first place. Big Jim finds Pauline in her old art studio, and tries to convince her that he's a changed man. Melanie hears about Julia and Barbie's plan for the egg. She's worried, so Barbie takes her outside for a chat. Through their conversation, we find out -- they're related! Barbie grew up hearing his parents argue over a woman whom his father was once involved with ... and they'd had a daughter. Melanie is his half sister! Over at the school, Norrie and Joe meet Hunter; he knows exactly who they are (from social media), and they find out he's behind the Hounds of Diana website. Hunter also knows about the egg -- and after overhearing that it's missing, offers to help find it. Back at Big Jim's house, Junior sees his mom alive and well -- but his happiness is short-lived, hearing Sam's come back with her. After Big Jim and Junior leave, the egg begins to glow and vibrate -- and Pauline grabs her head in agony!
Julia, Rebecca, and Barbie meet with folks at the diner; Tom is on board with their plans, but Andrea insists on hearing from Big Jim. Naturally, Big Jim wants to take control. Barbie wonders: maybe sending him to Zenith first really is the best idea -- especially if they force him to initially leave Pauline and Junior behind. At the police station, Hunter and Joe rig a radar gun to find the egg; downstairs in jail, Phil (Nicholas Strong) overhears everything. Big Jim goes home to find Pauline furiously painting; she's having a vision and sees "the end of everything." Hoping to protect her, Big Jim locks her in her studio and starts searching for the egg. But when he finds it in his shelter, it zaps him clear across the room!
Melanie decides to give the egg to Barbie and Julia, but Joe and Norrie find it first -- along with Big Jim, who threatens to shoot Joe if they don't hand it over. Junior locates Sam at his cabin and beats him to a pulp; just as he's about to kill him with an ax, the vision of Angie reappears. Sam can't die; he's got a part to play in the Dome's plan. Junior takes one final swing, but purposefully misses Sam's head. In the meantime, Hunter is looking at some suspicious emails on his phone ("Confirming arrival in Chester's Mill") when he hears Big Jim take Joe and Norrie down into the tunnels. Norrie won't throw the egg over the cliff, so Big Jim knocks it out of her hand; suddenly, an earthquake!
There's now no sign of the egg at the bomb shelter -- and suddenly, Melanie passes out. The continuing quakes also allow Phil to escape. Julia and Barbie bring Melanie to Sam's cabin; despite his injuries, he offers help. Hunter manages to save Norrie as she climbs out of the tunnels, just as Barbie and Julia enter the school. A disheveled Phil runs past them and into the locker; Barbie and Julia follow and come to the cliff. As they look down, they realize that the trench now has a bottom -- their way out is gone! And Phil, who threw himself over, is dead.