"Marathon: The Patriots Day Bombing" airs Monday, Nov. 21 on HBO. (HBO/HBO)

The Tsarnaev brothers’ two homemade pressure-cooker bombs, which detonated within moments of each other near the finish line of the 2013 Boston Marathon, claimed three lives and injured about 260 other people. 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 row.

The story is easily summarized, but the underlying meaning of its aftermath remains up for grabs. For a variety of reasons, including the deepest well of civic pride, the usual parameters of magnitude seem not to apply in Boston. What would be regarded as a routine occurrence of terrorism in other countries is, as far as Bostonians are concerned, on par with the same simmering affronts and lingering wounds as the far deadlier Sept. 11 attacks or the 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 the film (airing Monday night) dutifully examines the motivations and actions by the Tsarnaevs and the “Boston Strong” movement that sustained the city through its grief, “Marathon’s” emotional heart rests in its sympathetic portraits of three families that were at the finish line that day.

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 recoveries as anywhere near complete, impressing upon the viewer 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, who were 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 that 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 is also willing to concede that platitudes only go so far.

You can only imagine what “Marathon” might look like to someone living in Syria, but that’s another of the unknowable mysteries of magnitude. Jessica and Patrick joined another family, who lost their son, in asking that Dzhokhar Tsarnaev be spared the death penalty and instead get a life sentence. Other victims understandably want to see Tsarnaev die. It’s another way “Marathon” artfully demonstrates just how personal our senses of ramification and closure can be.

Marathon: The Patriots Day Bombing (108 minutes) airs Monday at 8 p.m. on HBO, with encores.