Around this time last year, Michael Mayer harnessed lightning at the Atlantic Theater when he directed “Spring Awakening.” That show has since become the Tony-winningest musical since “Hairspray,” and Mr. Mayer has returned to the Atlantic with a new musical, this one with a score and lyrics by the revered country-folk artist Patty Griffin. “10 Million Miles” is less of a thunderclap and more of a gentle early-summer storm, churning up its share of drama while leaving the promise of warmer, suppler days in its wake. The mixed-up pair of Southerners created by Ms. Griffin and bookwriter Keith Bunin may be a decade or so older than the torqued-up adolescents of “Spring Awakening.” But they remain every bit as baffled by the pitfalls, both biological and self-inflicted, of desire. And while Ms. Griffin and Mr. Bunin take a few ill-advised shortcuts, especially near the rushed ending, their efforts join “Spring Awakening” and the recent “Passing Strange” in illustrating that engaging musical theater needn’t fear incorporating voices from the mainstream.

Derek McLane’s spare set is devoted largely to the red Mazda pickup truck owned by a yarn-spinning yokel named Duane (Matthew Morrison). He cajoles Molly (Irene Molloy), a former good-time gal struggling to clean up her act, into joining him on a road trip from Florida to upstate New York. Their romantic history is confined largely to a debauched weekend before Duane shipped out with the Army a few months earlier; he has already been sent home, owing to some bad behavior on the base. Molly is pregnant, we soon learn, and while Duane might be the father, this is by no means a certainty. (To repeat, it was a wild weekend.)

Regardless, the possibility of a Duane Jr. proves alluring to the heretofore heedless Duane, who gradually sells Molly on the idea of keeping the baby and settling down. But the trip north (depicted almost entirely via Jules Fisher and Peggy Eisenhauer’s crisp lighting design) is riddled with emotional potholes, and both characters are forced to reconcile what seems right with what seems smart in a world where “everything before is gone or / Going somewhere.”

That struggle can be found at the center of untold country songs. And Ms. Griffin, whose work has been covered by everyone from Emmylou Harris to the Dixie Chicks to Reba McEntire, certainly knows her way around the idiom. While she tries — and largely succeeds — to filigree each song with a few musical or lyric idiosyncrasies, her ingratiating, rootsy score stays largely within the recognized bounds of well-crafted adult country music.

And while much of the score is pulled from Ms. Griffin’s back catalog — yes, it’s a jukebox musical — the songs feel natural, as if they were invented for these characters in these situations. (Mr. Bunin’s otherwise crisp book occasionally errs in padding the scenes with information that Ms. Griffin’s lyrics render superfluous.) In fact, with its banged-up truck and its wary but hopeful heroine, “10 Million Miles” could be described as a long country song.

Ms. Griffin’s knack for telling character details extends to the dozen or so people Molly and Duane meet along the way. These mechanics, spinsters, and gamblers are all played by Skipp Sudduth and, making a memorable New York stage debut, Mare Winningham. Even when “10 Million Miles” devotes questionable amounts of time to these fringe characters — for example, giving the show’s second song to a waitress and gas jockey who never reappear — it’s hard to quibble about Ms. Winningham’s numerous opportunities to show off her lived-in alto and miniaturist gift at delineating her various characters.

Not even this narrative asymmetry deters Mr. Morrison, who blends an acute sense of his character’s flaws with a remarkably assured tenor. In a pivotal early song, Duane tries to convince Molly that she is worthy of keeping the child she had been planning to give up for adoption. His pitch includes this enigmatic question:

What do you wish you were?
Do you wish you were the silence on the moon?

In the brief instrumental vamp between these two lines, however, Mr. Morrison cracks his neck. Twice, first to one side and then to the other. This could not be more counterproductive to the soft-spoken seduction Duane is plying here, but it’s absolutely true to his character’s overpowering restlessness. Mr. Morrison’s flirtatious, cocksure performance is filled with similar touches, each one contributing to a memorable depiction of an amiable, none-too-bright, heedless young man.

That small-bore level of precision, alas, is not matched by his costar. In terms of emotional clarity, vocal prowess, and even diction, Ms. Molloy’s performance rarely rises above the serviceable. Molly’s fate lies at the play’s jagged center, and yet despite receiving some of Ms. Griffin’s sharpest lyrics — a haunting tribute to the Virgin Mary stands out — Ms. Molloy consistently relies on a distancing array of vocal cracks and swoops.

She is also hampered by a slapdash finale, in which Ms. Griffin and Mr. Bunin gloss over a seemingly irreparable act of betrayal with a quick fast-forward to happier days. This is the first time that Mr. Mayer’s otherwise firm directorial hand fails him, and the resulting plot twist feels unearned while vitiating much of Molly’s turmoil.

“Spring Awakening,” for all its galvanizing power, and “Passing Strange” encounter similar troubles as they near the finish line, as does “Rent,” the granddaddy of modern-day rock musicals. The more literate, less bubble-gummy strains of popular music tend to traffic in darker sentiments: “I must confess there appears to be / Way more darkness than light,” Duane sings with one of his gearhead cronies near the end of “10 Million Miles.”

Squaring this tendency with the narrative conventions of musical theater — or, more to the point, what the creators continue to interpret as those conventions — remains a stumbling point for the genre. But the effort remains a noble and potentially transformative one, and even if it rarely reaches the heights of its aforementioned forebears, “10 Million Miles” represents another worthwhile stab at that elusive target.

Until July 15 (336 W. 20th St., between Eighth and Ninth avenues, 212-239-6200).