Harry Shearer makes for an unusual guide to post-Katrina New Orleans. Unless you’ve been reading his contributions to the Huffington Post, you’re more used to seeing him strumming an instrument in Christopher Guest mockumentaries or hearing one of the many characters’ voices he provides for The Simpsons. His quite hallowed spot in American comedy doesn’t prepare you for his role here, as director of an earnest political documentary in which he literally walks viewers through the problems he’s discussing

You don’t expect a comic like Shearer—even one as given to bone-dry bon mots as he is—to take on a deadly serious topic. Even less do you imagine he’ll deal with it in admirably straightforward fashion. Shearer only resorts to humor on a few occasions, mostly provided in some round-table interview segments titled, “Ask a New Orleanian” (hosted by John Goodman operating at maximum sarcasm level). While Shearer who has a house in New Orleans) puts himself front and center in the film’s opening sequences, he then steps back to let those who know what they’re talking about (and more frighteningly, those who seem not to, regardless that they’re in positions of authority) flesh out his thesis. This can be boiled down to the following: with regard to the New Orleans levees system, the U.S. Army Corps of Engineers is little more than a dysfunctional pipeline for government pork, using outmoded technology with little or no forethought or oversight.

Pushing this point are a couple of researchers who tried to assess what happened to the levees that burst after Hurricane Katrina blew through the city in August 2005. Professors Bob Bea, from the University of California, Berkeley and Ivor van Heerden of Louisiana State University, both came to the city with a team of other well-informed and well-meaning experts to figure out the cause of the disaster. It didn’t take them long to come to the conclusion that the flooding of the city wasn’t a natural disaster, it was man-made.

If this thesis isn’t news now, the details of it are still horrifying. As van Heerden and Bea make painfully clear in their lengthy, patient explanations, the levees were badly constructed by the Corps with substandard materials. (The film includes a helpful computer-mapped animation showing a bird’s-eye viewing of where and when the floodwaters spread through the city.) This fact, they contend, led directly to the levees bursting not because the hurricane waves overtopped them (as was originally claimed and is still widely believed), but because they were undermined and collapsed. That’s what building with sand gets you. While this would seem basic knowledge to just about anybody, not only do the Corps engineers not get it, Shearer and his subjects argue, but they also continue to deny any failure on their part and furiously discredit all critics.

This much is obvious in the interviews with some Corps employees (who, with one great exception, seem to be competing with each other for how to throw up the biggest bureaucratic smokescreen). Worse, as the film looks to the future, such interviewees are especially frightening, as they reveal the Corps is rebuilding the New Orleans levees system using essentially the same outmoded and shoddy practices that led to the problem in the first place.

The further that The Big Uneasy ventures into that territory, the more obvious it becomes that Shearer has uncorked a subject almost too big for this film. If even a third of what his guests are alleging is true, then a vast proportion of the nation’s infrastructure—having been built by a government agency where project managers are barely supervising massive pork-barrel constructions—is in dangerously bad shape. Here the film’s dogged determination to cover its topic thoroughly, as well as deliver a sharp riposte to those who think that simply abandoning New Orleans is either desirable or possible, is impressive.

Throughout The Big Uneasy, Shearer’s voice (as well as those of narrators like Brad Pitt, Jennifer Coolidge, and Wendell Pierce) offers calm rationality, thankfully free of actorly indignation. Any thinking person watching this film will be nearly overcome with agitation at the shortsightedness and blind stupidity on display. He or she will walk away knowing that New Orleans isn’t ready for another hurricane, and unless people actually force those in power to start thinking smart about how to build a city that will survive. Think: Amsterdam.