“Slay the Dragon” begins with a subject that might seem counterintuitive for a documentary on gerrymandering: the Flint, Mich., water crisis. The movie lays out a timeline of state legislative actions that led to the decision that contaminated the city’s water supply. It persuasively argues that the crisis never would have happened without gerrymandering, which had allowed legislators to shield themselves from voters’ wrath.

Connecting the dots between Flint and gerrymandering isn’t new; that case has been made elsewhere by the journalist David Daley, a consultant on the documentary and the author of “Ratf**ked: Why Your Vote Doesn’t Count.” For anyone who has read a book on gerrymandering or followed the legal fights over the matter, “Slay the Dragon” won’t necessarily offer much that is new. (The Supreme Court ruled last June that federal courts had no power to rectify partisan gerrymanders, a process in which state legislative majorities redraw voting maps to help their political side ensure control.)

But the film, directed by Barak Goodman and Chris Durrance, does a skillful job of distilling a complicated history. It recounts a Republican campaign to flip statehouses in the all-important election of 2010, a census year after which legislators could redraw boundaries. It tags along with Katie Fahey, a grass-roots activist, as she pushes for a ballot measure in Michigan that, in 2018, gave redistricting authority to an independent commission. And it dissects the extreme nature of partisan gerrymandering in Wisconsin — graphics show how sophisticated, data-driven mapmaking guaranteed that, even in the event of a Democratic landslide, Republicans would maintain power in the State Assembly — and its policy consequences.