The normally clandestine world of the Shin Bet, Israel’s secret service, is revealed with remarkable transparency in The Gatekeepers.

A thought-provoking documentary by Israeli filmmaker Dror Moreh, The Gatekeepers examines Israel’s decades of strife-ridden post-World War II history. Moreh tells the story through historic film footage and, most of all, six former heads of the Shin Bet, all of whom sit for the director’s camera and answer his tough questions.

A film in Hebrew with English subtitles, The Gatekeepers is being released in a time of heightened tension between Israel and the Palestinians who live in the Gaza Strip, the West Bank and East Jerusalem.

The Six-Day War of 1967, the war that made Israel the occupier of land inhabited by Palestinians, is the film’s point of departure. A recurring theme in The Gatekeepers is the likelihood that occupiers always become, to some degree, oppressors.

But there’s nothing black and white about The Gatekeepers.

“One man’s terrorist becomes another man’s freedom fighter,” says Yuval Diskin, Shin Bet’s director from 2005 to 2011.

Disillusion among the former directors is another theme. As a boy, Diskin recalls, he became obsessed with the prospect of a brutal conquest of Israel by Arabs. Another Shin Bet chief, Ami Ayalon (1996-2000) recalls his happy, post-Holocaust boyhood in the new nation of Israel. As a child he was told of a wise old man in a room at the end of a long corridor who always was watching over him.

The man Ayalon alludes to is David Ben-Gurion, first prime minister of Israel. But when Ayalon, the most articulate and dynamic of the film’s ex-Shin Bet chiefs, goes to that corridor and to that room as a grown man he finds no one there.

As dramatic as Ayalon can be, the film’s darkest moments belong to Avraham Shalom, Shin Bet director from 1980 to 1986.

In the film, Shalom, a chief said to be a ruthless bully, a man feared in his own country, has the look and demeanor of a jovial old grandpa. But this was a man who personally ordered the execution of the two captured and bound Palestinian terrorists following a 1984 bus hijacking between Tel Aviv to Ashkelon.

Eventually forced to resign, Shalom says his lesson from the incident was that politicians can’t be trusted and the press had best be kept in the dark.

“If he hadn’t come,” Shalom says of the reporter who got the story of the terrorists’ executions, “no one would have known.”

The Gatekeepers also covers the devastating damage caused to Israelis by Palestinian terrorists, especially a wave of suicide bus bombings in the 1990s.

A deadly serious and detailed examination of and meditation upon the Israeli-Palestinian conflict, The Gatekeepers makes no attempt to find a silver lining. But maybe the movie can start some useful discussion about the ever-festering stalemate in the Middle East.