This may be the most obvious observation of all time, but it must be said: “Reacher” works because of Reacher. The hit Amazon Prime Video series, based on the novels by Lee Child and currently wrapping up its fourth season, is a vessel for star Alan Ritchson and his remarkable physique, which his drifter character puts to use by clobbering numerous bad guys across the United States in a variety of inventive ways. (Before Ritchson, the eponymous action hero was played in two movies by Tom Cruise, who may lack his successor’s bulk but made up for it in star power.) There’s a difference between formulaic pulp that’s disposable and the kind that’s a comforting delight, and on “Reacher,” that difference is Ritchson.

All this to say is that Frances Neagley (Maria Sten, also of “Reacher”) is no Jack Reacher, and therefore the new spinoff “Neagley” is nowhere near as fun to watch as its flagship show. The most enjoyable scenes by far in the eight-episode season are the first few, when Reacher stops by to pass the baton to his ex-military comrade, now a private investigator in Chicago. Reacher helps Neagley foil a kidnapping attempt; then the two attend a Cubs game for some bestie bonding where Reacher’s mass and Neagley’s glower makes them stick out like sore thumbs. Cute!

But then Reacher splits off for his next adventure and Neagley is left to shoulder an entire show on her own. Co-created by Nick Santora and Nicholas Wootton, “Neagley” begins with the heroine’s childhood friend Tommy Crane (Jhaleil Swaby) stripping naked and walking calmly in front of an L train. The mystery his death introduces proves flat and uninteresting, with none of the buzzy elements stuffed haphazardly into the plot — cults! Big Pharma! maimed penises! — executed with enough verve to inject any life into the proceedings. And while Sten performs capably in the (sporadic, perfunctory) action scenes, her taciturn protagonist declaring that “I don’t have trauma” can’t reverse psychology the viewer into genuine curiosity.

That’s because Neagley is a collection of hard-surface, soft-heart tropes that never achieve independent momentum. Some shortcuts to emotional weight even feel counterproductive: in addition to Tommy, Neagley gets both an ailing father — an ex-cop who showed her the ropes — and a couple of mentees as loved ones with built-in stakes. But it’s hard to watch these thinly sketched relationships and not think of “Reacher,” which partly works because its hero is so free of roots or baggage. One of that show’s insights is that such exposition can get in the way of pure, brainless entertainment; it’s odd to watch a spinoff discard such inherited wisdom.

Other details simply fail to inspire much intrigue. Neagley doesn’t like to be touched, a quirk that doesn’t become any more effective a hook for how often characters use the condition’s medical name (haphephobia — the more you know!) when discussing it. “Neagley” tries to pad out the investigation into Tommy’s death with a look at Neagley’s own past, but neither her backstory nor that of her reluctant partner Hudson (Greyston Holt), a Chicago PD detective attempting to distance himself from his criminal family, make much of an impact beyond filling space.

Even Echo Canyon, the shady communal farm where Tommy spent his final days, is oddly banal for a messianic cult. “Neagley” sometimes flirts with comedy in the form of leader Lawrence Cole (Damon Herriman), a dime-store Manson who forces his followers to listen to his bad rock covers and unironically quotes Depeche Mode in declaring himself a “personal Jesus.” (Lawrence’s financial backer Pierce, played by Matthew Del Negro, gets his own chance to gnash his teeth when he informs a victim that “watching you die, I’m hard as a rock.”) But besides these flashes of fun, the show proves reluctant to lean into either camp or distinctive detail. Lawrence’s chief deputy Judith (Rebecca Liddiard) has awfully composed hair and makeup for a hippie who’s left polite society; street kid Keno (Jasper Jones), who Neagley recruits to infiltrate the farm, has an immaculate six-pack despite not having access to a hot shower, let alone a gym. The viewer remains acutely aware they’re watching professional actors impersonate vague stereotypes of cult members rather than an immersive take on the genre.

In light of this ersatz feel, the constant invocations of Neagley’s military record for authenticity ring hollow. (When she reminds a scene partner that “I’ve been in combat” halfway through the season, it’s hard to resist shouting back: “We know!”) The rough outline of what happened to Tommy is apparent within a few episodes, and without that source of suspense, “Neagley” quickly loses steam. With “Reacher,” the show’s namesake has become a reason to watch in and of himself. That’s simply not the case here.

All eight episodes of “Neagley” will premiere on Amazon Prime Video on Sept. 16.