It started with a rat trapped inside Theo Anthony’s outdoor trash can.

“Absolutely, what you see is really how it came about,” the Baltimore-based filmmaker says of his first documentary feature, “Rat Film,” which opens Friday in Baltimore, New York and Chicago. “I had come home one night, and I heard this sound in my trash can. So I whipped out my cellphone and just started filming.”

That brief footage, of the trapped rat desperately trying to jump a few inches higher than conventional wisdom says a rat should be able to jump, kicks off an 80-minute rumination on Baltimore’s decades-long battle against its unwelcome rodent population, and some disturbing parallels Anthony discovered with the ways city leaders have tried to deal with various urban situations.

“Rat Film,” which had its local premiere at May’s Maryland Film Festival, is alternately amusing and troubling. A parade of rat-centric characters troops through it, including one man who loves them so much he happily lets them climb on his head while he plays a flute; an exterminator who turns out to be a closet philosopher; a guy who hunts them down with a blowgun and almost religious fervor; and a homeless trio with a surprising, and decidedly gritty, musical bent.

It also features a history lesson that begins in the early part of the 20th century, when Baltimore became the first city in the nation to pass a residential segregation law, restricting blacks and whites to certain sections of the city (such laws were struck down by the U.S. Supreme Court a few years later). The movie ends with an alternate take on the city’s future that almost makes sense, in the most unsettling of ways.

His film developed, Anthony says, as he delved into two areas of research — Baltimore’s rats and some unfortunate attempts at urban planning — and realized they dovetailed in ways he never suspected they would. On the heels of the residential segregation legislation, for example, Anthony discovered that parts of the city, often poor and rat-infested, were “redlined” by the insurance industry, forever labeling some areas as both downtrodden and undesirable. And in the 1940s, he also discovered, one byproduct of social and medical research being done on some of these areas was the discovery of a better type of rat poison by a researcher at Johns Hopkins.

“It was like all these really strange coincidences that, when you start to really dive into it, you realize that they’re really not coincidences, that they’re really linked in their conceptual subject matter,” Anthony says.

Anthony, 27, grew up in Annapolis and started coming to Baltimore regularly when he was about 14 or 15, mostly to attend Dan Deacon concerts (not coincidentally, Deacon provides the eerie, electronic soundtrack for “Rat Film”). A graduate of Ohio’s Oberlin College, he’s now a full-time artist and photographer, with a resume that includes a handful of short documentaries and music videos.

He hadn’t planned to make a film about rodents and social planning his first feature, Anthony says, but the more research he did, the clearer his path became.

“I didn’t have any expectations, I didn’t know if anyone was going to see this film,” he says. “Rat Film” is 95 percent him, Anthony says, put together in his attic and room. What started as a five-minute short grew into something far bigger and more complex. His handiwork premiered at Switzerland’s Locarno Film Festival in October 2016, and was subsequently picked up for distribution by Cinema Guild.

“Rat Film,” Anthony stresses, is no conventional documentary. In one instance, for example, Anthony recruited actors to try their hands at rat fishing, attaching bait to the end of a fishing line and casting down alleyways, hoping to reel in a rodent or two. Rat fishing, he notes, is a real thing, an “urban myth” based in reality, and his actors really are doing it — even if it’s by design, rather than inclination.

(Anthony defends such a re-creation, while acknowledging that he, perhaps, should have somehow labeled it as such. “Being transparent about re-creating a myth is a different conversation than, ‘Should you be able to re-create it in this way,’ ” he says.)

Even more fundamentally, Anthony says, “Rat Film” reflects not so much history, or sociology, or social criticism. It does, he stresses, reflect the “personal journey” he undertook as a result of his research. Audiences are free to take from that journey whatever they want.

The film “presents itself as just one of many possible investigations into our city’s history,” he says. “It was a really personal journey. It’s not really an objective history of Baltimore, nor does it try to be. It’s a very personal investigation, and reflects my trying to understand my own role in that investigation.”

“Rat Film” opens Friday at the Stavros Niarchos Foundation Parkway, 5 W. North Ave. mdfilmfest.com.