NBC's "Chicago Fire" did not exactly generate a lot of heat when it launched last fall.

The drama about the truck and rescue squads of a fictional Chicago firehouse lacked the moody darkness, charismatic antiheroes and explosive violence of critical darlings such as "Breaking Bad," "Homeland," "Boardwalk Empire," "Dexter" and "Sons of Anarchy." The show's retro vibe and focus on heroism were decidedly more "old school" than "new cool."

The template also appeared a bit too close to those of "Rescue Me," "Third Watch" and other recent series centered on emergency workers. The cast largely consisted of unknowns; the most recognizable faces were a supporting actor from "Sex and the City" and an actress who starred in the horror-torture epic "Hostel Part II." There was more buzz for the show's 10 p.m. Wednesday rival "Nashville" on ABC.

But as the season winds down, "Chicago Fire" has quietly emerged as one of the few bright lights in NBC's troubled prime-time lineup, outdistancing series with higher concepts and bigger stars such as "Smash," "Revolution," "The New Normal" and "Go On."

While "Nashville" has lost steam, "Chicago Fire" is averaging more than 8 million viewers a week, up from its premiere audience of 6.6 million, and its pairing with veteran "Law & Order: Special Victims Unit" has formed a solid two-hour block for the network.

NBC has not said whether any of its new series will be back in the fall. But the future of "Chicago Fire" is discussed with cautious enthusiasm.

"This show is very successful, and we feel really good about it," said NBC Entertainment President Jennifer Salke.

Added executive producer Dick Wolf: "We're very optimistic."

The momentum of "Chicago Fire" is particularly gratifying for Wolf: It marks a success unconnected to the "Law & Order" brand he created more than 20 years ago. It also marks a significant thematic departure: Though the "Law & Order" mother ship rarely explored the personal lives of its core characters, the personal lives of the "Chicago Fire" characters are center stage.

"We have a very delicate balance between action and soap opera," Wolf said. "We thought this has a pretty good chance of working. It goes back to 'It's the writing, stupid.' The show is the purest example of retro TV, or comfort food."

The formula of "Chicago Fire" has been consistent: high-octane action sequences where lives are at stake mixed with volatile human drama. Lt. Matthew Casey (Jesse Spencer) has complex family issues and lingering feelings for his former fiancee.

Lt. Kelly Severide (Taylor Kinney) is recovering from a painkiller addition. Paramedic Gabriela Dawson (Monica Raymund) is pining after Casey even though she's starting a romance with trainee Peter Mills (Charlie Barnett). Fellow paramedic Leslie Shay (Lauren German) is a lesbian who can't seem to cut ties with a former girlfriend who has just given birth to her husband's baby.

Executive producer Matt Olmstead, the showrunner for "Chicago Fire," said of the characters: "These folks are together 24 hours on, 48 hours off. They're a dysfunctional family who are forced to get through it. Then there are these calls which bring this spike of adrenaline, and everyone has to pull together."

The relative lack of hoopla as the season began actually worked to the show's advantage, Wolf said.

"We were constantly trying to find the right ingredients to put in this souffle, and we had the luxury of flying under the radar so we were able to go through our first-season growing pains without much notice," he said.

NBC's underperforming "Smash" even gave an unwitting shout-out to "Chicago Fire" early in the season. In a scene showing the theatrical songwriting team of Julia (Debra Messing) and Tom (Christian Borle) walking in Times Square, the two were upstaged by a gigantic poster for "Chicago Fire" looming in the background.

Olmstead and the writing staff strive to give relatively equal weight to the characters in the large ensemble, which includes Eamonn Walker ("Oz") and David Eigenberg (Steve in "Sex and the City"). Kinney and "Hostel Part II's" German, whose characters are unlikely roommates, seem to have sparked the most interest among viewers.

But the most prominent star, and the key ingredient behind the appeal of "Chicago Fire," according to Wolf, is Chicago.

"This is America," he said." If you set this show in Los Angeles, people would say, 'Oh, come on.' But this is about the heart of the country. It's real."