“Heaven Knows What” has its Madison premiere at the UW-Cinematheque, 4070 Vilas Hall, at 7 p.m. Friday. R, 1:34, three stars out of four. FREE!

Brothers Ben and Joshua Safdie make films about people that we wouldn’t really want to be around in real life, and keep us uncomfortably close to them, force us to really look at them, until we see them.

“Daddy Longlegs” was a drama about a harried divorced dad so bad at parenting that his neglect borders on abuse. In “Heaven Knows What,” we’re attached to the hip of a 19-year-old homeless junkie, Harley (Arielle Holmes), caught in an endless and vicious cycle of addiction. When the movie starts, she’s panhandling (“spanging” in street parlance) to get enough money to buy razor blades to slit her wrists. You don’t get much lower than that.

Harley is threatening suicide to appease her cruel boyfriend Ilya (Caleb Landry Jones), who believes she has cheated on her. Maybe she has, maybe she hasn’t. Her life is so geared towards the obtaining of and using of drugs that everything else is just means to an end. We see Harley begging for spare change, shoplifting 5-Hour Energy drink bottles, stealing people’s mail — anything that will get her closer to that next fix. Being a homeless junkie is hard full-time work, as it turns out.

After Ilya cruelly drops her, Harley pinballs from one man to the next. There are many moments of horror in her life, but a few grace moments as well, such as when a dealer acquiesces to give her a ride on his motorcycle. Or there’s one sudden moment of loveliness, a wintry early morning when Harley and her latest companion are walking through an empty New York, and “Claire de Lune” swells on the soundtrack. But the reverie doesn’t last, and as Harley’s life goes back to grubbing for a fix, the soundtrack goes back to a nervy, looping synth score, following the cycling pattern of Harley’s life.

“Heaven Knows What” is a hard movie to watch, but feels steeped in authenticity. That’s because Holmes was in fact a homeless junkie, discovered by the Safdies, and the screenplay is based on her own experiences. She has an entrancing face that can harden into a predatory blade or soften with need, and it’s the humanity that we see in her that keeps us connected to her story. All the other actors in the film are her real-life friends, except for Jones, an actor who has appeared in an “X-Men” movie, for heaven’s sake. The fact that he fits seamlessly in among these real people on screen is a testament to the commitment of his performance.

In the end “Heaven” is something of a tragic love story, with Harley hopelessly wedded to the cruel and careless Ilya. In a voiceover late in the film, she rhapsodizes about how elegant and patient he used to be, and says “Everything I am today came from you.” Sadly, that’s probably true. And somehow, by the end of the film, we’re not looking into Harley’s eyes anymore. We’re looking through them.