The concept of a hostage thriller based around disdain for the health care system is already dubious. And at times, “John Q.” plays like “Dog Day Afternoon” crossed with a human resources seminar. But thanks to a convincing central performance by Denzel Washington, this implausible, preachy movie ends up a moderate crowd pleaser.

Washington plays John Q. Archibald, a factory worker who’s been demoted to part time because of cutbacks. Despite being awakened one morning by the sight of his car getting repossessed, his home life is still pretty wonderful. His supportive wife Denise (Kimberly Elise) is working full-time and his 10-year-old son Michael (Daniel E. Smith) is excelling at baseball, even though his dream is to become a body builder.

Denzel Washington plays a blue-collar worker compelled by the health care system to take the law into his own hands in John

When Michael collapses during a Little League game, everything changes for the Archibalds. Once at the hospital, the chief cardiologist (James Woods) and head administrator (Anne Heche) reveal that the child’s only hope is a heart transplant. However, the Archibalds’ insurance won’t cover this “elective surgery,” and the medical staff demands a down payment of $75,000 before they’ll even put Michael’s name on the transplant list.

As his son’s blood pressure continues to fall, John’s anger at the bureaucracy rises. Ultimately, an armed takeover seems to be his last resort, and the doctors and patients of an emergency room become unwilling participants in his plan. Or as the tagline proclaims, “The hospital is under new management.”

“John Q.” is the type of good-hearted fantasy where the little man is unyieldingly righteous and the system always corrupt. Conversations inevitably turn into arguments involving health care reform and insurance/medical policies, while the characters are forced to spout statistics at a time when profanity would be a more likely response.

Luckily, Washington is there to ground this HMO-phobic movie. The best moments in “John Q.” occur during the earliest scenes. In daily life at home, in the car or at church the cast playing the Archibalds has the kind of comfortable interaction that makes the audience believe it’s watching a real family.

That’s not necessarily the case with the physicians and police, who make the kind of fatuous professional decisions that only happen in movies. Their actions are often so absurd (such as the tactics of the final surgery) that it’s a credit to the talent of the supporting actors that the film doesn’t devolve into parody.

But even the Oscar-calibre cast can’t camouflage the stock nature of these roles. Robert Duvall plays a veteran police negotiator named Frank Grimes (a “Simpsons” reference?) who is principled and compassionate. Meanwhile his superior (Ray Liotta) is incessantly yelling, playing to the TV cameras and displaying general incompetence.

Then there’s the typical blow-dried newscaster (Paul Johansson) who is “on to the hot story” and committing unethical acts so he can scoop competing stations. “This is my white Bronco,” he declares. These peripheral characters are all so one-dimensional that they seem to thematically exist outside of the main story.

Director Nick Cassavetes (the son of legends John Cassavetes and Gena Rowlands) has proven himself skilled with actors his 1997 film “She’s So Lovely” was a tour de force for Robin Wright. He certainly coaxes fine work from Washington, who has at least double the dialogue and screen time as in his Oscar-nominated performance for “Training Day.”

But Cassavetes also proves to be either intimidated by or infatuated with his leading man. Frequently the filmmaker is afraid to cut away from moments in which the star is emoting. A bedside encounter where John “says goodbye” to his son lingers minutes beyond necessity. The movie could easily have been 15 minutes shorter simply by trimming (not deleting) existing scenes.

The maudlin music on the soundtrack doesn’t help the pacing. The tunes (new Patti LaBelle and Stevie Wonder numbers) often seem to be piped directly from the Muzak of the hospital’s waiting room to the soundtrack.

Even if “John Q.” brings some legitimate attention to the frailties of America’s health care situation, it’s still difficult to feel inspired when being preached to so shamelessly. At times the audience feels like it has a gun to its head.