Once there was a lost, fatherless boy named Spencer Stone, who worried his mother and antagonized his teacher.

Spencer liked to stare out the window during math class. His teacher recommended ADD meds.  

Instead, his mother moves her son to a Christian school. There he makes regular visits to the principal's office for minor infractions. But he also feels the stirrings of faith, a sense that he has been created for a special purpose.

Spencer’s high school years are no different. And once the better students head to college and the workforce, he doubts whether he can find a way to fulfill his special mission to “help people.”

Clint Eastwood’s new film, “The 15:17 to Paris,” is the story of Spencer Stone, the lost, fatherless boy, whom the would now honors as a hero. In 2015, at the age of 23, the U.S. airman helped overpower a heavily armed terrorist on a high-speed train to Paris, saving countless lives.

How did Spencer defy the odds and become a hero? Why didn’t he become a casualty of our selfish, increasingly violent throwaway culture?

In the wake of the Florida school shooting and other mass killings, such questions have only become more urgent. While Spencer is not a misfit, precisely, his struggle to become a man for others not only proves decisive for his own future, it actually saves lives.   Why did he choose this path, while other young men with a similar profile begin a downward spiral?

Eastwood doesn’t attempt to provide tidy answers. Nor does he deal explicitly with the increasingly common problem of mass shootings by alienated young men.

Rather, this clean, simple narrative celebrates the relationships that give us hope, as well as the mysterious, unseen forces that lead us back from the brink of despair.

During the darkest days of Spencer’s youth, it is his mother’s fervent love, and the friendship of his two longtime buddies — Alek Skarlatos and Anthony Sadler — that keep him afloat.

And beneath the surface, a nascent faith slowly molds his conscience, helping him distinguish God’s voice in the cacophony of modern life

“Lord make me an instrument of your peace,” prays Spencer, when he is alone in his bedroom as a middle school student. The Prayer of St. Francis is repeated later in the film. And by then Spencer has shed his couch potato persona and joined the armed forces, stepping up his game for the first time in his life.

Military discipline introduces a series of setbacks. Spencer still wants to “help people” but fails to secure the post he wants, and he briefly toys with quitting.

Slowly, he gains confidence, and his desire to serve others gets a boost from his superiors, who exhort new recruits to embrace “the struggle of life. … Don’t try to take any short cuts. Do what you know is right.”

Spencer’s life moves forward, and a whirlwind trip to Europe with Alek and Anthony offers a chance to celebrate his accomplishments.

The three young men visit St. Peter’s Basilica at the Vatican and tour the canals and museums of Venice. Yet throughout their time together, Spencer perceives an unseen force “catapulting” them forward.

“I don’t know, man, ever just feel like life is pushing us toward something, some greater purpose?” he asks his two friends.

Alek and Anthony look puzzled. But the audience is invited to ponder a question that has long inspired the young to set aside their selfish desires and probe their mission on earth.

As Spencer scans the horizon, there is no trace of complacency. He wants to be fully prepared to meet his destiny, whatever that may be.

On Aug. 21, 2015, the three friends board a high-speed train to Paris. Spencer will be the first of them to tackle the Moroccan-born terrorist, Ayoub El Khazzani, who is armed with an assault rifle, a pistol, a knife and a trove of ammunition.

“Spencer, go!” Alek calls out. His friend runs straight at the man, whose weapon had jammed. Then, as Spencer helps restrain the terrorist, the man pulls out a box cutter and nearly severs the airman’s thumb. Despite the injury, Spencer rushes to help another passenger, who had been shot by the terrorist and has lost a good deal of blood.

Later, during interviews with reporters, Alek acknowledges that their aggressive response to the terrorist had been a “choice,” and they were lucky to be alive.

The 15:17 to Paris is based on a book written by the three men, who also play themselves as adults in the film. Their performances are low-key, but believable. 

During a ceremony honoring Spencer, Alek and Anthony, then-President François Hollande of France celebrated their heroism.

“One need only know that Ayoub El Khazzani was in possession of 300 rounds of ammunition and firearms to understand what we narrowly avoided — a tragedy, a massacre,” said Hollande.

“Faced with the evil of terrorism, there is a good, that of humanity. You are the incarnation of that.”

Indeed, ordinary boys -- even those who spend way too much time in the principal's office -- can grow up to be extraordinary. The flame of faith, and the steadfast love of family and friends can help a late bloomer become a real hero, a man for others.