Let me start by saying that I am really baffled as to where the producers and writers are going to come up with another 13 episodes to (in theory) wrap everything up next year. What a mess!
The Dome has been the home of seriously questionable science this season, of the sort that causes me to want to bang my head against the wall. Yes, we can have blood-red acid rain that is stopped (nigh immediately) by blowing a little high-pH solution into the air. We can have the dome start rotating so that it drags -- drag? The dome has drag? -- frigid air from the upper atmosphere down to the ground, darn near freezing everyone to death. (If it was pulling that much air around, you'd expect more wind, I'd think. But that think part is important, because you shouldn't.) Oh, and we almost killed everyone by unleashing a deadly influenza virus to thin out the herd, because that trick's going to work so well.
These are the things that bother me, not dead girls returning to life or mysterious passages into and out of the Dome. Well, that and the fact that the outside of the Dome seems to be controlled by the men in black helicopters. Or private industry-owned black helicopters. Or something. But clearly they are evil.
Heck, I don't even mind the odd relationships -- Barbie is the dead girl's younger brother and they're both the kids of the evil power baron in Zenith. Sure! No problem! I've seen Once Upon a Time. Everyone's related!
Anyway, in the season finale, a new way is found that will probably lead out of the Dome -- and since the Dome has decided to start shrinking, it seems like a good time to be leaving, "probably" or no.
But first it's time to thin the herd a little. One of the four who originally found the egg / meteor twenty years ago resulting in Melanie's death (one), mortally wounds Pauline (two), is killed himself by Big Jim in revenge (Lyle, three), leaving only Sam (four) left alive. Rebecca (who nearly killed everyone with the aforementioned flu before repenting) administers an overdose of morphine at Pauline's request, but is not clever enough to make sure that Big Jim doesn't find out, so Big Jim kills her too. And then, because the Dome doesn't bring Pauline back to life, he decides it's now time to go on a killing rampage and take out anyone who is a "Friend of the Dome", mostly Julia and Barbie. In the pursuit of Julia's death, Big Jim lures her to the home of another resident who he then kills just for the general heck of it, but mostly to show Julia that he is bad ass. Julia manages to escape by ramming a knife through Big Jim's foot which, if nothing else, will slow him down.
So the town is headed down a hole, but in the general confusion and earthquakes, Big Jim, Julia, and Junior are left behind on the wrong side of an underground ravine which grows wider just in time to make sure that Julia and Junior are staying on the wrong side -- fortunately for Julia, Big Jim is not yet in the hole, so she's safe for as long as it takes him to get down there. (Hey, Barbie, there's an underground ravine. You brought a chain-link escape ladder, but you couldn't bring some planks or plywood to help the more infirm residents across?)
Anyway, Barbie's hand print causes the dead-end of the tunnel to open up, revealing the vanished (last episode) Melanie who tells them all to "Follow me. We're going home."
I can only conclude they're all going to spend next season on an island with a crashed passenger jet liner.
Or something like that.