Global warming has caused catastrophic flooding and turned both the water and the atmosphere poisonous, resulting in the collapse of civilization and the death of most of humanity. Fifty years later, civilization is slowly trying to reestablish itself. Cars -- called "belchers" -- are illegal and the government sends out bounty hunters to catch drivers and bring them in, dead or alive. All this is explained in an opening title card describing an interesting, Mad Max-style movie. Unfortunately, it has very little to do with this one. This movie is a Western that has no need to be post-apocalyptic (though it is) and makes almost no sense.
Gage is indeed a bounty hunter, but she hunts bandits, not drivers. She gets paid in clean water, which is used for money, and -- if she's lucky -- silver, which is the key ingredient in the filter masks people have to wear against the toxic atmosphere. Her mentor is Doc, an acerbic, grizzled former bounty hunter who took up doctoring after suffering a leg injury that left him with a limp, and who John Hannah plays exactly like Dr. Gregory House. Gage wants to go after the biggest bounty of them all, Jackson, an outlaw who's building a town of outlaws and threatening to take over the New Montana Territory. House tries to talk her out of it as certainly suicidal. Gage won't give up but does agree that, rather than a frontal assault, she'll assume the identity of a bandit she recently killed to infiltrate Jackson's town and gather intelligence.
She goes to the town, successfully convinces everyone she's the dead outlaw, hangs out in the bar for a couple hours, learns absolutely nothing useful that I could discern, and leaves. However, on the way out, Jackson himself calls out to her and asks her to stay (why?). She does, for reasons as inexplicable as her early departure. She hangs out in town for a couple days (why?). Meanwhile, back in New Montana, the corrupt sheriff decides to organize a posse to take down Jackson. House tries to talk him out of it as certainly suicidal, but the sheriff ignores him. The sheriff successfully sneaks up and has Jackson dead to rights, except Gage calls out a warning and saves his life (why?), leading to the deaths of the posse and the capture of the sheriff, who Gage then shoots before he can ID her. Gage then hangs out in the town for several more days, ignoring several obvious opportunities to shoot or capture Jackson (why?).
Jackson then reveals that he's located an abandoned silver mine that he wants to reopen, but he needs laborers to run it. Fortunately, there's a convoy of pilgrims driving through the area in an RV (wait, isn't that illegal?). Jackson organizes a raid, only to discovered that the pilgrims have hired an armored car (I can't even???) to escort them. The guards massacre most of the men Jackson brought with him and have the bandit leader dead to rights until Gage blows up the armored car (why?), turning the tide of battle, capturing the pilgrims (why?), and saving Jackson's life (why?). Impressed by this, Jackson offers her a 5% stake in the silver mine (way more than he's giving henchmen who've worked with him for years), which she negotiates up to 15% (huh?). Finally, Gage decides to take out Jackson, captures him, and is immediately taken prisoner by his guards. Rather than stringing her up outside of town with all the other bounty hunters he's killed, Jackson has her nailed into a coffin and thrown off a cliff, then doesn't bother to check the body (why?).
Gage comes to, drags herself back to House's house, then drops dead on his doorstep. He performs CPR, and then there's the inevitable getting back into shape training montage, after which Gage and House decide to launch a frontal assault on Jackson's town just like they could have in the first ten minutes of the movie. First, though, they go to the silver mine, kill the one guard (???) and free the pilgrims. Then they attack the town, killing all the henchmen (then why didn't they just do that at the start?). Jackson, however, escapes for the inevitable boss fight in the silver mine, while House stays behind for the equally inevitable deputy boss fight in the main square.
