When last we visited Chester's Mill, Junior (Alexander Koch), Rebecca (Karla Crome), and Sam (Eddie Cahill) had just discovered a tunnel under the very high school locker where Angie was murdered. Hoping to find Lyle, Junior climbs down and goes exploring ... and Sam follows. At the Sweetbriar Rose, Melanie (Grace Victoria Cox) confides in Julia (Rachelle Lefevre): she kissed Joe (Colin Ford) in an effort to feel "normal," but it backfired. Why is she here? Does it have something to do with that glowing egg she'd found right before her death in '88? Julia tells Melanie that she dropped the same egg in the lake minutes before Melanie reappeared. Maybe the Dome chose them both for a reason? Over at Big Jim's (Dean Norris), Joe and Norrie (Mackenzie Lintz) argue, and Joe leaves in a huff. Back at the diner, Julia tries to convince Big Jim to give the town back to its people -- but Big Jim once again shows his true colors. "Half of this town is mine," he says, "and don't you forget it." Just as Big Jim confronts Barbie (Mike Vogel) outside, Rebecca radios in; they've found a tunnel, and Sam and Junior are hunting Lyle down. As Barbie drives off, a dust storm kicks up around town. Now far underground -- past where the Dome should've stopped them -- Barbie finds Sam and Junior. They see Pauline's journal -- but as Junior tries to grab it, it sets off an explosion, leaving Barbie and Sam trapped!
Everyone's alive, and Junior runs to get help. Sam is convinced the explosion was Lyle's doing, but Barbie isn't buying it. Since they can't get out the way they came, the two decide to push on. Big Jim's made his way to the high school; Julia and Melanie show up too, and Junior explains the whole situation. Big Jim believes they have a bigger problem -- the dust storm. Rebecca thinks the dirt could clog the Dome, which would choke off their oxygen; they've got to get water into the air as soon as possible. Junior tells Julia that Lyle knows about the new "Four Hands." Fearing for everyone's safety, Julia tells Junior (and Melanie) to find the kids. Down in the tunnels, Barbie and Sam find a fork in the "road" and split up. Big Jim confronts some townsfolk in the diner, and asks for a vote; much to his chagrin, they side with Julia and decide to wait the storm out. Sam tries (and fails) to sneak up on Barbie and knock him out. As the two argue about Sam's history with Melanie, Barbie trips over a cliff into a seemingly endless cavern -- but Sam manages to save him.
Junior, Melanie, and Norrie find Joe at his old house, and they decide to go to the lake to find answers. Rebecca believes the only way to get Sam and Barbie out of the tunnel is to blow up the cave-in, and Julia begrudgingly agrees. Back in town, after Joe’s friend Ben (John Elvis) almost dies from a dust-induced asthma attack, Big Jim finally gets everyone on board with his plan. Meanwhile, Sam comes clean to Barbie about the night Melanie died; in a sense, he let Lyle push Melanie into that crater -- and the memory is ruining him. Big Jim and the town raise a giant windmill and shoot water through it; just as Rebecca believed, it settles the dust. The teens and Junior take a boat out onto the lake and hold hands; suddenly, the egg begins to glow under the water. While Rebecca and Julia get explosives ready, Barbie notices the wounds on Sam's shoulder and puts two and two together -- he killed Angie! Sam admits his guilt; he can't live under the Dome for one more second, and if killing the new "Four Hands" will bring it down (like Pauline's journal seemingly shows), he'll do it. At Joe's house, the quartet touches the egg, which emits pink stars. Some form the shape of an obelisk; Junior realizes he's seen the same image in his dream, and Melanie thinks it's a monument from her hometown, Zenith. Rebecca's explosion rocks the tunnel just as Sam makes Barbie's decision for him and jumps off the cliff! While Big Jim spins Julia's recent decisions in an unflattering light to the townspeople at the Sweetbriar Rose, she finds Barbie safe ... but where's Sam gone? Is he actually dead?