Early in the film we meet Michelle (Mary Elizabeth Winstead,) a troubled young woman fleeing a fight with her fiancee. After a car accident she wakes in Howard's (Goodman) Underground Bunker, which seems to be protecting all inhabitants from radiation or the government or Russia or aliens, and in this day and age most of us would wake to that news and be like, "lol, what's for lunch?" but this is 2016 and Michelle is still concerned with Howard's vague details and erratic moods. He tells her that his wife has abandoned him (shit, I wonder fucking WHY?) and shows Michelle a picture of his young daughter, whom he says "is no longer with us." Also joining them underground is kindly townie Emmett, who broke his arm trying to beat the door to get *in* to play board games and eat spaghetti with John Goodman. So, I mean, yes, we're assuming that whatever is outside is unpleasant.
The two are busy making Hazmat Suits out of a duck shower curtain when Howard confronts them with a vat of acid. Emmett, playing the brave older brother, tells Howard that it was all his idea to steal the gun and Michelle knew nothing, so Howard shoots him and shoves his body in the acid. Then of course Howard just leaves the vat of acid open for Michelle to knock over onto him because why not? and then the acid eats though the cord of a lamp and starts a fire. She collects her duck suit and escapes, even though acid-faced Howard is chasing her and trying to stab her. She manages to get out before the bunker explodes and the air is just fine, take that, John Goodman. Unfortunately, then some alien garbage happens.