“Slay the Dragon” is the most important political film of the year, and it may prove to be one of the key political films of the decade. It’s a documentary about gerrymandering, and offhand it would be hard to think of a subject less sexy — or a phrase less inviting to audiences than “a documentary about gerrymandering.” But that issue, more than any other (maybe even more than Donald Trump, though it’s all working in tandem), is the subject from which American democracy now hangs. By a thread.

And “Slay the Dragon” is an incisively made and morally suspenseful film, at once chilling and stirring. It deals with gerrymandering on a human level — as the endgame in the fight for democracy — and it shows us what the resistance to it now looks like: a bunch of ordinary citizens, with no power beyond what the Constitution gave them, building the crusade for voter rights into the ultimate liberal holy war.

A lot of us have a concrete picture in our minds of how gerrymandering works: a state map cut up into skewed geometric shapes, each representing a voter district designed to favor one political party over the other. (For years, the Democrats used it as flagrantly as the Republicans.) The word dates back to 1812, when it merged the name of the Massachusetts governor Elbridge Gerry with the last half of “salamander” (because yes, that’s what the districts resembled). So gerrymandering, sleazy and unrepresentative as it is, has been with us for a long time. Most of us accept it as a cruddy built-in element of the system, like rush-hour traffic, television commercials, or the electoral college.

But “Slay the Dragon,” directed with investigative force by Barak Goodman and Chris Durrance, reveals that gerrymandering was really in its minor-league phase right up until 2010, when the Republicans, with a grandly sinister intent they made good on, launched a full-scale initiative that might have been called Gerrymandering 2.0.

The new Republican strategy was to carve up the map into districts so ideologically ironclad — and so snaky and corrupt and arbitrary in their Lego-structure-from-hell design — that the districts could lock in elected officials more or less indefinitely. The new districts resembled shapes out of some twisted children’s drawing game: Ink Blots for Junior Dictators. When voter districts are mapped out this extremely, their effect is to place a force field around the status quo. And that’s more than a threat to democracy. It is, in effect, one-party rule — the end of democracy.

The film opens with the water catastrophe in Flint that began in 2014 (it’s not over), and while that subject has been amply covered, “Slay the Dragon” traces the disaster directly — and definitively — to gerrymandering. Once the Republicans, in 2010, felt that they were impervious, because they had redrawn the Michigan map into perpetual majority districts, they no longer saw themselves as accountable to the voters. They could do anything they wanted. So they appointed non-elected “finance managers” to deal with the state’s fiscal crisis, and it was these managers who decided to tap the Flint River for the city’s water supply.

It’s mandated that voter districts be redrawn once a decade, during the census year (1990, 2000, etc.). Two years after the election of Barack Obama, Republicans launched a nationwide redistricting campaign with a stroke of malignant genius: They realized that the key political races were the state legislatures, which had the power to redraw voting districts. So they tapped deep-pocketed corporate donors, from Wal-Mart to the Koch brothers, to throw $30 million at state campaigns.

It was an unprecedented sum for that level, but a drop in the bucket for what the Republicans were getting. The money funded the kind of relentless high-level negative advertising that’s generally reserved for national contests, and the local Democratic candidates didn’t know what hit them. They suffered major defeats — in Michigan, North Carolina, Wisconsin.

As time went on, a number of Democratic candidates in those states actually won more votes. But it didn’t matter. The way gerrymandering works, even when you win the popular vote, it’s meaningless if your voters are crammed into just a few districts (a technique known as “packing”) or sprinkled into the opponent’s districts (known as “cracking”), dissipating those numbers. Packing and cracking are the bow and arrow of gerrymandering. And they’re now utilized with the aid of computerized data analysis, which predicts voter patterns with a precision that was scarcely possible before the digital age.

Once officials are elected with little to no accountability, they can introduce things like voter ID laws or (as Gov. Scott Walker did in Wisconsin) the elimination of collective bargaining for public employees. These are all ways of heightening the effects of gerrymandering; they reduce actual elections to a fig leaf on fascism. It’s so out in the open that Chris Janowski, the Republican strategist who founded Project REDMAP (Redistricting Majority Project), was happy to sit down and talk with the filmmakers about what he’s doing. He comes off as a Steve Carell character, but what he’s owning up to is insidious. It’s all “legal” (even though any sane court would find it unconstitutional), and that, from the Republican point-of-view, is the sick beauty of gerrymandering. It makes the stealing of power look legitimate.

“Slay the Dragon” isn’t all ominous Republican plots. Katie Fahey, a young woman from Michigan with no background in politics and no connections, gets on social media to try and jumpstart a grassroots movement, and lo and behold, she does it. She names it Voters Not Politicians, and throughout the film we see it gather steam. Fahey is a stirring figure, because she starts off seeming like a passionate ordinary person, and by the end of the movie her fervor and rhetoric have become cleansing enough that I started to see her as a born politician (even though that doesn’t seem to be her intent).

And in Wisconsin, a consortium of activists mount a case that, in 2017, goes all the way to the Supreme Court. This section of the movie is a real pulse-pounder. Watching it, I couldn’t remember why I didn’t remember the outcome — until I was reminded that the Supreme Court (with Justice Kennedy still in place) didn’t rule in either direction; the justices wound up punting the case. That left the door open to the possibility of a future anti-gerrymander ruling, a door that slammed shut the moment that Brett Kavanaugh was confirmed.

But that doesn’t mean the war is over. It means that it now needs to be fought on the state level, the way Katie Fahey did (and — spoiler alert! — she won). As a documentary, “Slay the Dragon” has the power to galvanize audiences by planting this issue on the map in a bold new way. The movie makes you see how the fight over gerrymandering is an essential battle for the American system. It’s a fight for the very shape of freedom.