StarSolidStarSolidStarSolidStarHalf(3.5 stars)

In the intriguing, marvelously inventive documentary “The Infiltrators,” a young Dreamer named Marco notes that whether he’s called “illegal” or “undocumented” doesn’t really matter. “They’re just other words for being afraid.”

But he doesn’t stay scared for long. As a member of the National Immigrant Youth Alliance, a band of creative and indomitable activists, Marco goes undercover at the Broward Transitional Center in Pompano Beach, Fla., where hundreds of nonviolent detainees wait to hear if they will be released or deported. Posing as a naive undocumented immigrant, Marco gets himself arrested by the Border Patrol and sent to Broward, where he proceeds to organize low-priority detainees, advising them on navigating a privatized bureaucracy that otherwise threatens to swallow them whole. Soon he is joined by Viri, who engages in similar activities on the women’s wing of the institution.

As a real-life thriller, “The Infiltrators” is the kind of tense, politically engaged drama that would normally lend itself to an earnest big-Hollywood message picture starring Gael Garcia Bernal, Antonio Banderas and Demian Bichir (with Sam Rockwell as the no-nonsense warden). But in the impressively resourceful hands of filmmakers Alex Rivera and Cristina Ibarra, this absorbing and timely story has found its ideal form. Constructed from interviews with the real-life participants in the events being chronicled — which happened in 2012 — the film uses fabulously staged reenactments to revisit what transpired inside the detention facility, where the detainees are treated like prisoners, and where messages are passed back and forth with the ingenuity of the most sophisticated tradecraft.

There are more than a few pulse-pounding moments in “The Infiltrators,” which also could have been titled “The Exfiltrators,” as one of the film’s chief protagonists, an Argentine landscaper named Claudio Rojas (played in the dramatizations by Manuel Uriza), endures nail-biting news of whether he will be released, and to where.

Reminiscent of a similar hybrid docudrama called “American Animals,” Rivera and Ibarra’s film doesn’t fictionalize fact as much as bring it to vivid emotional life, allowing viewers to enter a world that is usually kept at a politically and psychologically safe distance. Some of the most harrowing scenes in “The Infiltrators” center on Viri, who when she gets herself detained must submerge her own smart, self-possessed persona to convince the guards that she’s illegal. “It wasn’t enough that I had to turn myself in,” she says through tears. “I actually had to . . . become the stereotype we advocated against.”

In the years since the events of “The Infiltrators” transpired, the issue of immigration has been exponentially demagogued, racialized and toxified; Rivera and Ibarra remind us that, as imperfect as U.S. policy was back then, at least it was open to incremental but important changes like the Dream Act. This invaluable film gives an unforgettable face to people who for years have been hiding quietly and productively in plain sight — in our neighborhoods, offices, restaurants and classrooms. “A year ago I was reading Kafka,” Marco says as “The Infiltrators” gets underway. “Now I was living it.”

Unrated. Available May 1 via streaming at afisilver.afi.com. Contains some obscenity. In English, Spanish and Farsi with subtitles. 95 minutes.