The Tsarnaev brothers' two homemade pressure-cooker bombs, which detonated within moments of each other near the finish line of the 2013 Boston Marathon, killed three and injured about 260 others. Seventeen people lost limbs. A campus police officer was shot and killed in the ensuing manhunt, as was the elder Tsarnaev. The younger brother was wounded, charged, tried, found guilty, and sentenced to death.

The story is easily summarized, but the meaning of its aftermath remains up for grabs. For a variety of reasons, including the deepest well of civic pride, it's different in Boston. What would be regarded as a routine occurrence of terrorism in other countries is, as far as Bostonians are concerned, on a par with the far deadlier Sept. 11 attacks or Oklahoma City federal building bombing. Even last summer's mass shooting at an Orlando nightclub, which claimed 50 lives, may not carry the resonance of Boston in the long run. But who knows? It's a terrifying world.

Ricki Stern and Annie Sundberg's moving HBO documentary Marathon: The Patriots Day Bombing, produced with help from the Boston Globe, can be viewed as a thoughtful study of the lessons of magnitude.

While Marathon (airing Monday night) dutifully examines the motivations and actions by the Tsarnaevs and the "Boston Strong" movement that sustained the city through its grief, its emotional heart rests in its sympathetic portraits of three families who were at the finish line that day and were seriously injured.

There are the Corcorans - Celeste and her husband, Kevin, with their adult children Sydney and Tyler. Celeste lost both legs and Sydney suffered critical injuries. And there are the Norden brothers, Paul and J.P., who lost their right legs. Marathon is careful not to present the stories of their recovery as anywhere near complete, stressing that an event like this is never finished for the victims.

That aspect is most evident in the story of Jessica Kensky and Patrick Downes, who open their lives the most to Stern and Sundberg's camera. Jessica and Patrick, newlyweds in 2013, each lost a leg in the bombings. Patrick's recovery and rehabilitation is quicker than Jessica's. She undergoes numerous unsuccessful surgeries to save her other leg, in a valiant attempt to live up to all the Boston Strong-ness playing out around her, putting a primacy on full recovery and moving on. She yearns desperately for the kind of media-friendly ending her husband presents, particularly when he runs in the 2016 marathon to much acclaim, including her tearful, undying support at the finish line. It was easier, she says, to play the part of strong survivor than to be honest about her setbacks.

There's a subtle but important message here that prefers real life over tidy conclusions. These people will never be the same, even with all the care, support, donations, and attention they've received. Marathon does what it can to make you feel good about the human spirit, but it also shows that platitudes go only so far.

"Marathon: The Patriots Day Bombing" (108 minutes) will air Monday at 8 p.m. Philadelphia time on HBO, with encores.