If you’re like me, you’ll have an uneasy feeling in the pit of your stomach for most of “The Gatekeepers,” a new documentary, opening Friday in South Florida, about Israel’s national security agency, Shin Bet. What you may feel, particularly if you’re even slightly well-versed in 20th century Israeli politics, is the physical embodiment of perpetually encroaching dread; the possibility, and frequent certainty, of bombings, uprisings and retaliations hiding in every nook and cranny of the only democracy in the most volatile region on Earth. In other words, you’ll feel a bit like Israel itself.

“The Gatekeepers” is the antithesis of a breezy entertainment, the opposite of an audience pleaser. It’s virtually humorless, and its outlook for the future of the Middle East is uncompromisingly bleak, its experts too knowledgeable for their pessimism to be countered. If “Zero Dark Thirty” painted a ruthless intelligence quest with a satisfying payoff, “The Gatekeepers” series of interviews with the upper echelon of its own CIA presents no such catharsis, instead viewing the future through jaundiced eyes, where the nation they represent is no closer to peace with its Arab neighbors than it was in 1967.

But “The Gatekeepers” is also the epitome of an essential film, one that all Americans should see, and which should be required viewing at every military base, intelligence bureau and university in the country. Because, while it’s hard-to-watch and realistically gripping, it’s also exhaustively educational. Director Dror Moreh received heretofore unprecedented access to six of the former heads of Shin Bet, as they reflect on the biggest terrorist attacks and intelligence threats to Israel over the past 35 years. There is extensive time, buoyed by occasionally gruesome stock footage, devoted to events such as the 300 bus hijacking, the PLO, the first and second intifadas, the Oslo Peace Accords and the revolt from Israel’s religious orthodoxy that followed, the assassination of Itzhak Rabin, and the Shin-Bet’s own premeditated attacks on Hamas leaders. “The Gatekeepers” presents a vital history lesson of the United States’ strongest ally through the prism of six men and one organization, and it does so fluidly and brilliantly.

Moreh can be a tough interrogator, asking all the right questions and the proper follow-ups and challenging his subjects when appropriate. Their candidness, in response, is striking. Some grow defensive and uncomfortable, others accept responsibility for the organization’s missteps (including one awful example of collateral damage to innocent Palestineans). But all are respectful and wise, helping Moreh – and the film’s viewers – to comprehend the complex moral calculus that goes into the work they do.

Watching “The Gatekeepers” is a reminder that while our current debate about the use of American military drones is a new one, the questions it raises about the possibility of indiscriminate killing are not unique to it. And in this film, the Shin Bet leaders offer clear-eyed insight – if not the elusive binary solution we could hope for – to help steady our own wobbly moral compass.

“The Gatekeepers” opens Friday at Regal Shadowood and Muvico Palace in Boca Raton, Regal Delray Beach, and the Coral Gables Art Cinema.