Considering how many millions of people have watched CBS' juggernaut military crime drama "NCIS" in 21 seasons on network TV, you probably know Leroy Jethro Gibbs.

You know, the always-gets-the-bad-guy leader of the team of special agents portrayed in all his chiseled, salt-and-pepper glory by Mark Harmon for more than 400 episodes? The one with the penetrating glares, firm sense of personal morality and fewer words than most mimes?

Did you know there is even more to his story than two decades on TV could tell?

Well, at least, that's what CBS is banking on with "NCIS: Origins" (Mondays, 9 EDT/PDT; moves to 10 PDT/EDT on Oct. 21, ★★½ out of four). Set in 1991 with a fresh-faced Austin Stowell as a young Leroy (replacing Harmon's real-life son Sean, who played young Gibbs in "NCIS" flashbacks), "Origins" takes the tried-and-true formula of mixing patriotism, military culture and murders-of-the-week to the past. The twist here is a surprisingly good period soundtrack, which must've cost a fortune in licensing fees, and a neo-noir style to suit its melancholy young Gibbs, whose wife and daughter have just been murdered.

Cast interviews:Mark Harmon asked 'NCIS: Origins' new Gibbs, Austin Stowell: 'Are you ready for this?'

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Just as it worked in Los Angeles, New Orleans and Hawaii, the "NCIS" procedural recipe mostly fits into ye olden days of 1991. "Origins" is good enough for military work, if a little too self-serious. It doesn't help that its leading man is the most boring element of the show. But the writers spin up a good enough case to solve every episode, and the cast fits into neat, recognizable boxes. If you want more of the same but just different enough, well, CBS has done it again.

Our young U.S. Marine Corps Scout sniper-turned-special-agent Gibbs shows up for his first day at the San Diego NIS offices with bruised knuckles. (Astute viewers will remember that the titular federal agency used to be called "Naval Investigative Service.") In this office, he's the probationary officer given the nickname "probie", learning how to catalog evidence and trust his gut. While he works through his grief for his family, he helps put the bad guys away for any crimes remotely involving the Navy or Marine Corps. Just as in all "NCIS" series, there are a shocking number of them.

Stowell, mostly unknown aside from some TV work and a lead role in Lucy Hale rom-com "The Hating Game," clearly took notes on Harmon's many episodes of "NCIS." He maintains Gibbs' classic stoicism and prickly demeanor without much effort. He certainly smolders with every glare and has a jawline that could cut glass, so it's not hard to see why CBS cast him. If his Gibbs is impenetrable and shallowly defined, well, that's a fundamental flaw in the whole concept of creating a show around a character known for being annoyingly mysterious.

If Gibbs is the least interesting part of the Gibbs origin story, so what? The fine folks at CBS, including "Origins" series creators Gina Lucita Monreal and David J. North, know how to throw together a rag-tag team of appealing Navy cops in baseball caps and latex gloves. This time the caps just say "NIS" instead of "NCIS."

So to round out the cast we've got the talented, ambitious woman, Lala (Mariel Molino), who's skeptical of Gibbs' and his mental state while being a better investigator than any man on the team (Ziva, anyone?). There's Randy (Caleb Foote), with McGee-level earnestness and a silly nickname. Secretary Mary Jo (Tyla Abercrumbie) swoops in as the resident maternal figure. And don't forget young versions of "NCIS" favorites: agents Mike Franks (Kyle Schmid) and Vera Strickland (Diany Rodriguez), both here to help with the nostalgia play. Franks, now Gibbs' boss, has to rein in the young whippersnapper before his impulsive heroism gets them all in trouble. But Franks isn't too concerned. He doesn't play by the rules, either.

Clichés abound, and the stories can get a little hammy, but "Origins" also takes a whack at being the thinking person's "NCIS." Lala has to figure out how to navigate Franks' blatant sexism and office politics. Gibbs is seriously traumatized and hasn't dealt with it. Franks is wracked with guilt for letting the murderer get away in Gibbs' family's case. I'm not expecting any transcendent moments from the series, but hey, it is trying to be a little more grown-up than its predecessors, known for soapy twists and sometimes infantile notions of good and evil. It wins some points for effort.

So go ahead, enjoy the almost-as-handsome-as-Harmon Stowell and friends as they solve naval crime in the '90s. Enjoy the sometimes corny dialogue and "SNL" alum Bobby Moynihan, who shows up as a forensics tech. Feel comforted by the familiar, but just a little different.

And if this "NCIS" spinoff doesn't work for you, there will always be another one.