If there is such a thing as critical tumescence, then that's what I have for Showtime's "Masters of Sex." Should I feel embarrassed? Not at all.

"Masters of Sex" (which returned Sunday night on Showtime) is a fine example of why some of us are still happy to spend a little more on our cable bill. It treats viewers like grown-ups - grown-ups who are curious about sex of all forms. It is intelligent, witty, quick-paced and surprising; it is tragic without being emotionally devastating. Based on the life stories of noted sex researchers William Masters and Virginia Johnson (Michael Sheen and Lizzy Caplan), the show is a satisfying exploration of American culture on the verge of loosening its grip.

Among its premium-channel peers, "Masters of Sex" is the rare drama that honors completely linear movement. Its hours are full. It doesn't obfuscate key details or use quick-edit cuts as a tease. It doesn't raise existential crises and mysteries that it then refuses to answer.

Though the writing and acting are uniformly excellent, the themes can sometimes be too blunt, always reduced to this: What a bunch of uptight jerks and jerkettes we used to be when it came to the continuum of human sexual response, burdening ourselves with expectations that kept things in cruel order.

At times, "Masters of Sex" seems intent on demolishing any notion of the 1950s as the good ol' days. But because it has been conceived and produced in the context of our present-day culture wars, it also seems to be sending signals that we are maybe only halfway there in becoming the enlightened society we imagine ourselves to be.

For example, in one upcoming story line, Masters finds himself at odds with the angry father of a baby born with ambiguous genitalia. Masters also has to fight the instinctive response of his fellow doctors, who rush to "fix" the infant with the easiest surgical solution rather than the more complicated treatment that would spare the child a lifetime of identity grief.

Whether the real-life Masters was as attuned to the issues that would preoccupy us in 2014, "Masters of Sex" is nevertheless demonstrating a topical relevance in either era.

Sunday's episode picks up where we left off last fall, circa 1958, after Masters's disastrous presentation to his Washington University colleagues of the outre research that he and his assistant, Johnson, spent months conducting, in which participants copulated and masturbated under observation.

Though their anonymity remains intact, it's only a matter of time before Masters's neglected wife Libby (Caitlin FitzGerald) will discover that her husband was a participant in the study with Johnson, a divorced mother of two.

Masters is a portrait of a man ruled by contradictions and compartmentalizing, which Sheen plays to perfection. Regarded as the hospital's expert in helping couples make and deliver healthy babies, he is nevertheless coldly dismissive of Libby's maternal instincts and ignores their infant. ("Masters of Sex" has been alternately forthcoming and coy about who really fathered the child, if not Masters, who claims infertility.)

At his worst, Masters became so fixated on the study that he alienated Johnson, who went to work down the hall for the socially awkward yet fiercely intelligent doctor Lillian DePaul (Julianne Nicholson), a cancer-stricken gynecologist dedicating her final energies to raising awareness about cervical health.

Driven out of the university hospital, Masters must find a new professional home and a way to continue his research. Happily for viewers, this entails the return of former prostitute Betty DiMello (Annaleigh Ashford), in an altogether different guise.

Masters and Johnson continue their research on one another, which Johnson is starting to recognize as love, and Masters is only barely willing to admit is an affair. In a few weeks, "Masters of Sex" will deliver what I think is its best episode to date, when the two of them unwittingly discover a path to greater intimacy: role playing.

By pretending to be other people, Johnson at last discovers some of the demons locked away inside her difficult lover. These are marvelously written scenes, set in a hotel room in which the television is broadcasting the December 1958 fight between light heavyweight boxing champ Archie Moore and Yvon Durelle.

Learning a few boxing moves in her hotel bathrobe, Caplan continues to deliver a performance that is gorgeously sharp; only occasionally does it seem as if too much of the show's central theses are filtered through her character - enough so that she sometimes becomes less relatable.

This season also further explores the marital woes of doctor Barton Scully (Beau Bridges), who, against his friend Masters's advice, seeks electroshock therapy to cure his homosexuality. As Scully's heartsick but empathetic wife, Margaret, Allison Janney is doing some of her best work since "The West Wing."

More new characters are coming (played by Sarah Silverman and Courtney B. Vance, among others) and producers have hinted that "Masters of Sex" will soon shift the plot forward a few years; welcome news, since the more fascinating parts of the Masters and Johnson story take place five and 10 years down the road, when they finally make it out of the lab and onto America's nightstands and talk shows.

"Masters of Sex" airs on Sundays at 10 p.m. on Showtime.