For a decade, Hamas’s highest leadership had a mole: a young man whose family ties ran so deep in the organization that his deception was unimaginable. To Palestinians, Mosab Hassan Yousef committed the ultimate betrayal; to Israelis, he was the most prized possession of an intelligence apparatus that sabotaged the militant political group’s campaigns.

The turning of this suave double-dealer is told in the documentary The Green Prince, which traces the unbelievable transformation of Yousef from next in line to lead Hamas into a top informant, and the unlikely friendship that develops between him and his Israeli spy handler. The result is an intimate and unnerving dive into their relationship narrated by the two subjects. The film has already snagged an audience award at Sundance, and will open theatrically on Sept. 12 in New York.

Code-named “The Green Prince,” for the emerald flag of Hamas and his royal pedigree as the son of the group’s co-founder, Yousef passed intel from the highest echelons of Hamas to Israel’s internal intelligence agency, Shin Bet, between 1997 and 2007. In a simultaneous rise through the ranks, he became both the improbable heart of both the militant movement’s leadership and Israeli intelligence gathering.

He’d later be lauded for foiling a plot to assassinate former Israeli President Shimon Peres, and the locking up of all varieties of Hamas commanders, militia founders, and bomb makers. As he spied on his own family for a government they considered occupiers, these successes made him Israel’s most valuable secret operative.

Yousef is now a 36-year-old Christian yoga teacher living in Los Angeles, unable to ever return to his family and home in the West Bank city of Ramallah. He bears little resemblance to the undercover agent he was a decade ago. Physically, he’s lost his under-bite and baby weight, and ideologically, he’s ditched not only his religion, but turned his back on one of the world’s most notorious militant political organizations that his blood still ties him to.

In recent weeks, he’s made appearances on channels like CNN and Fox, urging Obama to support Israel, and asking “How many more wars will there be if Hamas is allowed to continue?”

But nearly 20 years ago, Yousef, the eldest son of Sheikh Hassan Yousef, a founder of Hamas and a top figure in the West Bank leadership, seemed destined to follow in his father’s footsteps. “Hamas was not just a movement to us, it was the family’s business,” he remembers in The Green Prince.

An instilled hatred of Israel had fermented in him over years of watching his father repeatedly arrested. When Yousef turned 17, he sought revenge, purchasing guns and ferrying them into the West Bank, unsuspecting that Shin Bet had already placed him under surveillance.

“The first day handling him was the first day of the end of my career,” remembers Gonen Ben Yitzhak, the Shin Bet agent who would spend the next decade working with Yousef until his eventual firing. (The Israeli shadowy intelligence services—Shin Bet is just one of three branches of the Israeli General Security Service—are revered and reviled worldwide for their controversial tactics that few other countries match in cunning and ferocity.)

Before they grabbed Yousef, only one other spy was known to have performed such high-level treachery for Israel. Decades ago, the foreign intelligence agency Mossad allegedly recruited Ashraf Marwan, son-in-law of Egyptian President Abdel Nasser, who would warn Israel on the eve of the surprise Yom Kippur war in 1973. He was “the greatest spy we ever had” until The Green Prince, says Michael Bar-Zohar, a former Knesset member who has been writing about intelligence community for 40 years. Marwan mysteriously died in 2007 not long after his identity was exposed by a journalist.

When Hamas emerged as an increasingly powerful threat at the turn of the century, “Mosab was a godsend,” remembers Bar-Zohar. During the young spy’s tenure, Hamas had become “the most ferocious enemy of Israel,” operating an underground terror network of bombings, kidnappings.

In the film, Yousef sits in a replica interrogation room, built with 30-foot-high walls, and narrates what happened next during the real interrogation. “He asked, ‘Will you work for us?’ I thought, ‘Is this guy out of his mind or what?’ I will never betray my father.” But, with the intention of going home and never following through, he agreed. He was thrown into jail to ensure no suspicions were raised, and it was there, fatefully, that Yousef observed the violence that he never knew existed in Hamas.

“I was looking at that picture [of the situation] and thinking, ‘This is my father’s project.’” It was, he remembers in The Green Prince, shocking. “I start to realize we are living a lie and people are dying because of this lie.”

So upon his release in 1997, Yousef agreed to a meeting with Shin Bet. “It crossed my mind, when I bring someone to a meeting I destroy his life,” his handler, Ben Yitzhak, says bluntly in the film. Shin Bet agents rarely unveil their identities, and Ben Yitzhak’s involvement in The Green Prince is unprecedented. A heavy-set man with a buzz cut, he narrates with an honesty that at times makes him seem like a monster puppeteer and at others reveals his humanity. “For me as a handler, my sources are somewhat like toys—this is a big game and I’m a player.”

What followed was a complicated, destructive game of cat and mouse, in which each side delicately wielded a deadly amount of power. It was also a dance, for Ben Yitzhak, to keep Yousef from himself. “You don’t talk about the fact that he’s actually a traitor,” Ben Yitzhak says in the film. “If you put the mirror in his face it would finish everything.”

***

Over the next decade, Yousef became Israel’s top intelligence asset, betraying the movement his father founded to provide Shin Bet with messages between Palestinian leaders, warnings of attacks, and assistance in stings that nabbed terror suspects. His involvement was a high-level secret, and a major coup for Shin Bet, which had just seen its reputation battered by the 1995 assassination of Israeli Prime Minister Yitzhak Rabin.

At the time of his recruitment, not much was known about the inner workings of Hamas, which had then kept its political and military factions more separated than it does today. Yousef’s intel was invaluable, even though, despite urging by Shin Bet, he refused to grow a beard or regularly attend mosque and never became an official member of Hamas.

He seems at once humbled by guilt—“I felt I took each one of them and put them in the altar and slaughtered them—all of them,” he says of his family—and pleased with his accomplishments, smiling slyly as he describes details of the double-dealings.

“This is the weakest creature that walks the face of the earth: people,” Bar-Zohar says. “You can never know who can become a spy for the enemy if you have enough means to pressure him, to blackmail him, to tempt him.”

In 2001, protests turned deadly in Israel, with a string of public bombings in both Tel Aviv and Jerusalem during the second Intifada. “I realized that cowards in the name of courage are leading children, women, and an entire nation to death,” Yousef says. He says he never did the work for money, but instead to rectify that destruction he observed. “You are risking everything and there is no celebration—I promise you, we never opened a bottle of champagne because we saved dozens of lives.”

His father turned to the person he trusted most: his eldest son. Yousef was tasked with arranging meetings and vetting guests. Secretly, he was pulling cloak-and-dagger stunts like switching out coffee tables at meeting spots with replicas bugged with microphones. In the film, he credits many of his actions to attempts to protect his father, once even writing him an anonymous warning letter that sent the elder Yousef into hiding.

“He gave me secrets…and I gave all his information to the Israeli intelligence, that was in all means betrayal, [but] in my world it was responsibility, it was saving his life and many other people’s lives,” Yousef says in The Green Prince.

It’s information that would be have been useful as the crisis between Israel and Hamas escalated this July. Though Yousef was active in the West Bank Hamas, Gaza would have been attainable infiltration.

“I don’t think the Shin Bet today knows as much as it would want about what’s happening in Gaza despite all [the resources] we have,” says Bar-Zohar, whose recent book examines the history of Mossad. “What we don’t know exactly is where are the leaders of Hamas and how they are protected—we know they are underground [near] the Shifa hospital in Gaza—but how do you get there? How are they protected? All this information could have been fantastic for us.”

He mentions in particular how Hamas’s capabilities greatly increased with an advanced network of tunnels that has seemingly caught Israel by surprise—signaling poor intel in the area. “I think when this war ends there’s going to be a lot of boards of inquiry in Israel.”

Shin Bet went to great lengths to ensure Yousef wasn’t exposed. At one point, Israeli special forces raided Yousef’s family’s house, launching a missile into the living room, all secretly orchestrated by Shin Bet to show that Yousef was a wanted terrorist.

But soon he and the agency’s leadership began to clash. Uninterested in playing a terrorist, Yousef encouraged his father’s newfound interest in a truce with Israel to the displeasure of the agency. The breaking point came when Ben Yitzhak was fired for violating protocol after allowing Yousef to take short getaways in Israel.

Promoting a false identity and conflicting alliances became too much, and Yousef left for America under the pretense of getting jaw surgery. Instead, he stayed and secretly applied for political asylum.

But his asylum was rejected, thanks to the blurring of lines that intelligence assets are often mired in. Israeli intelligence fingered him as a terror threat, hoping, he seems to think, to get him to return to the country and their control. Yousef had left the agency in 2007, just a year after Hamas swept the elections in Gaza, and they were surely desperate to bring him back. “Once you become connected with them, the secret services never, ever, ever let you go,” says Bar-Zohar. “It’s a Catholic marriage.”

***

Going public, Yousef thought, would fix this. His personal outing began with his father, who he called in jail. “I’m not who you think I am,” he told him. Yousef says the leader replied: “You are my son and you will continue to be my son, no one can take that away.”

Yousef then released his whole tale in a book, Son of Hamas, which was published in 2010. Though his earlier conversion to Christianity had been made public, his treason sent shock waves. “I’m going to become from a prince to a traitor immediately,” he thought. He didn’t hold back with criticism of the family business. “The Hamas leadership is responsible for the killing of Palestinians, not Israelis,” he told Haaretz in 2010. “I tell you with certainty that the Israelis care about the Palestinians far more than the Hamas or Fatah leadership does.”

As the world picked up the story, his father’s tune changed. A message smuggled from his jail described his son as a traitor and disowned him. “All of the things stated in his story about his activity against Hamas and ‘its fighters’ are a crude lie that has not even one [grain of] truth,” it read. (The elder Yousef, now white-haired, was released from jail in January.)

“The impact in Israel was tremendous, and also in the Palestinian and Hamas world, when they saw somebody whom they admired as a hero, one of the young leaders of the organization, turn out to be an Israeli spy,” Bar-Zohar says. In turn, he says, Hamas became even fiercer in its rebellion against Israel.

But things weren’t going according to plan for Yousef: He was alone, homeless, and had already lost his first bid for asylum. Ben Yitzhak, watching the case unfold abroad and sensing Yousef’s likelihood of being shipped back to Israel, where he’d be in great danger, decided to out himself—a move unheard of within Shin Bet’s top-secret ranks.

“We who know this shadow world become very cynical with age—this relationship in which one side relies totally on the other is kind of a business arrangement, but in this, very strangely, they became very close friends,” Bar-Zohar says. The Green Prince director Nadav Schirman echoes this. “I’ve rarely see two friends share such a bond of trust. It is truly inspiring.” Today, Schirman says Yousef is known to his ex-handler’s children as “Uncle Mosab.”

When Ben Yitzhak turned up at Yousef’s immigration hearing in 2010, Homeland Security dropped the case against his application. Suddenly Yousef was hailed as a hero by Israel’s Knesset, and lauded for his bravery by then-CIA Director James Woolsey.

“I will keep fighting the ideology that is behind terrorists because I know how they think,” he told reporters outside the courtroom. “I know that this is the real danger that is facing liberty, facing freedom, facing humanity.”

The tale was quickly slammed as Zionist propaganda by Hamas, and not all Israeli officials were convinced of his heroics, either. Former Shin Bet Deputy Chief Gideon Ezra, who served before Yousef’s time, called the tale “too good to be true.” And added, “there are hundreds of collaborators like him. He is not unusual. He just decided to write a book about it.” Many more, however, have spoken in support of his work.

In the film, Yousef says he hopes people in the region can come together like he and Ben Yitzhak did. At the moment, this wish seems unlikely. “The film tells the story of a relationship between two enemies who took the risk to trust one another, and through this they established the basis of a relationship,” says Schirman. “In the peace talks that collapsed it seemed that none of the sides were willing to take that risk to trust the other. Without this, it seems, that no relationship is possible.”

But there is something to be learned by the cloak-and-dagger world from Yousef and Ben Yitzhak. The recruitment, success, and later, unveiling, of Hamas’s most successful spy has left a lasting mark on intelligence gathering. Schirman says Shin Bet—which doesn’t benefit from a cold portrayal in The Green Prince—is asking its agents to go see the film. They’ll watch as Yousef, who once called Shin Bet handlers “the devil himself,” and Ben Yitzhak, who considered Yousef a plaything, become as close as brothers.

This means, the director says, that an Israeli-Palestinian duo has taught one of the world’s top spy agencies the value of remembering that those risking their lives in the field are human. In a conflict where both sides do their best to dehumanize the other, this cinematic reminder of collaboration and kinship is welcome, especially against the bloody barrage of current news reports.