Mark Wahlberg returned to his hometown to play Boston Police Department Sergeant Tommy Saunders in Patriots Day, the intense new film opening this week that follows the lead up to and manhunt after the Boston Marathon bombing in 2013.

Wahlberg, 45, produces as well as stars, which gives you some idea of how deeply personal the project is to him – he’s a native of hardscrabble Irish American neighborhood Dorchester -- since it unfolds on the city streets that he grew up in.

Although he’s playing a fictional composite character, Wahlberg’s remarkably assured performance is aided by Peter Berg’s skilful direction, which brings a you-are-there immediacy to the proceedings that piles on the tension without ever losing sight of the everyday people caught up in the maelstrom.

We know how the story begins.  April 15, 2013 was a picture-perfect Monday in Boston. That morning the 117th annual Boston Marathon kicked off, drawing massive crowds.

The race fell on Patriots’ Day, which commemorates the first battle of the Revolutionary War, and spectators had gathered along Boylston Street in the heart of the city in the hope of seeing the world’s top racers and their own loved ones crossing the finish line.

But at 2:39 p.m., two bombs detonated 12 seconds apart, turning a community day of celebration into a day of stark terror. The blasts killed three spectators instantly (one of them was eight-year-old Martin Richard) and they wounded more than 260 people.

What followed was unprecedented in the history of the city. The governor and local law enforcement effectively shut the Boston area down, enacting a form of martial law as police desperately searched for the bombing suspects, 19-year-old Dzhokhar Tsarnaev and his 26-year-old brother Tamerlan Tsarnaev.

Boston is, of course, an Irish American stronghold, and on the day Celeste Corcoran had joined her husband Kevin, daughter Sydney and son Tyler at the marathon finish line because her sister was running.

When the bombs detonated Celeste lost both legs. Her daughter Sydney also sustained critical injuries and later would suffer extreme Post Traumatic Stress Disorder, and Kevin had his world upended as he tried to make sense of what had happened to his wife and daughter.

From its opening scenes Patriots Day is told from Sergeant Saunders’ point of view, which represents the real life actions of several policemen as they grapple with the shocking aftermath of the explosions and the citywide manhunt that quickly follows them.

Most people already know the basic outline of the story, how the Tsarnaevs, who spent part of their childhoods in Kyrgyzstan but lived in the U.S. for a decade prior to the bombings, planned and carried out the attack on their own, completely unconnected to any Islamic terrorist organizations.

What we don’t know and what Patriots Day excels at is depicting the struggles of the ordinary people of Boston behind the headlines, the ones who had to contend with the blasts and their aftermaths.

Lending even more reality to the events, the film uses actual footage from the scenes and from the places where the police later staged live shootouts.  This turns out to be the most effective story telling device of all, because the heart-rending cries you hear from the real life footage will horrify you and leave you in tears of disbelief.

In less skillful hands Patriots Day could have been offensively crass, a rabble rousing Go USA propaganda exercise. But in Berg’s hands it becomes a story about ordinary people responding to acts of unimaginable cruelty and showing the kind of everyday real world heroism that we can all believe in.

Patriots Day is deeply respectful of its subjects, its treatment and its city: in other words, striking a very necessary balance between reality and entertainment.

“I took on the responsibility of making sure we got it right,” Wahlberg told the press at the premiere in Boston. “If anybody's going to be held accountable in this community, it’s me. I come here to visit my mom, I want to visit the Boys Club, I want to go the neighborhood. And I want to be able to hold my head up and walk the street with pride.”

For once the anxiety to make sure they got the tone right has paid off.  Wahlberg delivers his most heartfelt performance in a role that may be the most important of his career.

Patriots Day knows how vulnerable our freedoms make us in the U.S., but the film never argues for their curtailment. The only way to respond to the coldhearted attacks on what we stand for is to calmly reassert why we stand for them in the first place.

“Obviously, I’m very serious about all the movies that I make and I want them all to be successful, and I want them all to work and serve their purpose, but this is so much more important than anything I’ve ever been a part of,” Wahlberg told the press.

On the sidelines. Still from Patriot's Day.

On the sidelines. Still from Patriot's Day.

Faced with the horror of what happened to their city, Bostonians did what few expected: they responded with love and selflessness instead of bitterness. That response became known as Boston Strong, a defiant stand taken against the forces that had expected them to only cower in fear.

Irish American actress Michelle Monaghan plays Wahlberg’s wife Carol, bringing a rich layer of family ties to the story. The terrorists wanted to sow division and fear, but Monaghan and every character in the drama show us that they really brought the city together in defiance.

Not everything works, it must be said. There’s virtually no examination of the background and lives of the two attackers. What compelled them and why did they turn on the country that had given them new lives? In particular, what motivated them to plot to injure and kill as many innocent people as they could? These questions remain mostly unaddressed.

There is no excuse for their horrifying actions obviously, but some context other than that “they’re evil Muslim terrorists” would help (the soundtrack swells ominously every time they appear, and the shootout in Watertown adds blast bombs that overturn police cars and turn the scene into a suburban war zone, taking serious liberties with what really happened).

So Patriots Day doesn’t strain to avoid every hint of propaganda -- for a start the poster itself is red, white and blue -- and what remains is indeed a fine film.

But if we are ever going to tackle the root of the extremism that is directed at us we’ll need to understand more clearly that it’s done in defiance of the Islamic faith, not because of it.

Patriots Day ends unforgettably with a series of to-camera interviews with the actual men and women on the ground that day in Boston, including those who were personally affected by the two blasts in what is the most heart wrenching sequence in the entire film.

You don’t have to be a Hollywood celebrity with a lantern jaw to be a hero, the film reminds us. You just need to stand firmly on the side of love and peace.

Patriots Day opens in New York, Boston, and Los Angeles on December 21, and nationwide on January 13.

Mark Walhberg star of Patriot's Day.