This would be my quick take on some major plot points:
1. Opening Image to the Inciting Incident — The opening sequence in the abandoned town, the walk to the bridge, the loss of the four-year-old son. This sequence lets us know the world the family is in and the loss of the son lets us know how dangerous it is.
2. More on their world: We see the outside world around the farm and the inside world of the barn and the fact that Evelyn is pregnant. They are preparing to bring new life into the world. We see that Marcus is skittish and less than happy, and Regan is intelligent but troubled. We see that Lee expends a lot of physical and emotional energy trying to make that outside world safe (scene at the silo). Evelyn does the same with the inner spaces of the barn. (In ways, a traditional sorting of roles).
3. The fire from the broken lamp re-enforces how dangerous life is.
Act II is a series of scenes which serve to establish that:
a) the family has found a way to preserve the desire to keep living; to survive. The little dance scene between Lee and Evelyn serves this perfectly. The elaborate preparations necessary for when the birth of the new baby comes also serves this function.
b) kids will be kids. Marcus is afraid. Regan is dissatisfied with her role and she harbors conflicted feelings over her part in allowing the young brother to get the space shuttle.
c) scenes which establish that the overriding goal of Lee and Evelyn is to prepare their two children to live and survive — just to keep on going.
If I had to pick a mid-point, it would be the scene at the waterfall where Marcus asks his dad “you still love her?” Lee says of course, and Marcus says “you should tell her.” It foretells so much of what is to come.
I believe the end of Act II comes when, after Evelyn has given birth, she asks Lee “who are we if we can’t protect them? … you must protect them!”
Is of course the show down and the resolution. What is resolved? We see the fruit of the parents’ work with Marcus as he grows to be a courageous young man when he ignites the fireworks which gives them all a chance. We see Regan release her anger and she and her dad make peace and she finds her own special power and a belief in her intelligence. And Evelyn is there in the end to guide the two children through the final showdown.