At the end of the second episode of Showtime's 1950s-era sex research drama Masters of Sex, called "Race to Space," Lizzy Caplan's character, Virigina, stands in a dimly lit hallway and reads a children's book about a boy adrift in space. We hear the text aloud in voiceover while sad, slightly mysterious music plays. It's the final scene, the show signing off for the week on a strange and pensive note. This odd moment was the first indication I got from this series that it might have a bit more rattling around in its head, and its heart, than it initially seemed. This being Showtime, and the pilot having an obvious interest in attracting as many viewers as possible, in the beginning it seemed all too likely that Masters of Sex would rely on nudity and other easy titillation to hook viewers in, like so many a Showtime series before it, and stop there. But this second episode literary flourish, artsy and Mad Men-like (and who cares if it's copycatting, better Mad Men then, say, NCIS), suggested that the show has bigger and deeper aims. So I stuck with it, and after last night's stellar episode, the sixth, I'm very glad I did.

While there is still plenty of sexy stuff to keep the audience hot and bothered, the show has embarked on plenty of other emotional and psychological journeys. The past two episodes have maybe even been some of the most thoughtful and serious (in a way that the network never is) television that Showtime has ever done. That doesn't meant that the show has gotten too heavy, but there's a definite, and surprising, somberness to the show that's proving compelling. Masters of Sex isn't snide about sexual repression the way it could have been, making an easy joke out of the 1950s the way Pleasantville did (no knock to that delightful movie, though). Instead, really, it's very kind, humane, about people and their fears and questions and wants and wishes.

Last night's episode expanded the show's world a bit more, giving focus to Allison Janney's character, the sexually stifled — clueless, even — wife of the closeted university provost played by Beau Bridges. That may sound like a cliche, the sad wife of a secretly gay man not knowing how good sex can be, but her story was handled sensitively and gracefully. It certainly helps it was Allison Janney, bringing her brand of gentle but patrician dignity to a role that could have been predictably mousy.

All the women on this show, in fact, are terrific. Prior this, Caplan was largely known for her sardonic, decidedly contemporary work in Mean Girls and on Party Down. When I heard that she would be playing this role — an accidental, but determined, sex researcher who ended up fundamentally altering notions of human sexuality — it seemed like it would be a strange fit, too earnest for such an appealingly prickly comic actress. And indeed, in the pilot episode she is a bit stiff, affecting a formal voice that's uncomfortably put-on. It's a very Period Piece kind of a voice, the same that an actor in college might use when doing a Chekhov play or something. But since that episode, filmed a long time ago, Caplan has relaxed beautifully into the role, now giving one of the more forceful, but understated, performances currently on the air. She has a lot in common with Julianna Margulies on the equally strong The Good Wife, both possessing an almost saturnine quality that tantalizingly, alluringly hints at hidden depths. It's a pleasure to watch Virginia throw herself into her work (which is pleasure), because Caplan herself seems so invested in her own project.

Caitlin FitzGerald, playing William Masters's despondent wife Libby, has also bloomed wonderfully. With her blonde, glassy poise, Libby could easily be compared to Mad Men's Betty. (Same name and everything.) Both are married to stern, elusive career men, both burn with inner fire that is only barely being addressed. But the characters really aren't all that similar on closer inspection. Where Betty is often petty and selfish, at times even vindictive, Libby is genuinely decent. That might sound tedious, who wants to watch someone be good all the time, but the way FitzGerald and the writers are exploring Libby's emotional landscape thus far makes the character as surprising and engaging as she would be if she was sleeping around or telling other dangerous lies. The last two episodes in particular have given FitzGerald a lot of Big Stuff to grapple with — a miscarriage, which led to a drunken bit of sad make believe while on a Florida vacation — but encouragingly the show hasn't stepped in many potholes of too-familiar plotting. Libby is growing into a unique character, with her own set of values and desires, all delicately rendered by FitzGerald.

Really it's the main guy I have the most trouble with. It's not really a problem with Michael Sheen as an actor, it's just that Masters is drawn as such an intense and focused-to-a-fault kind of a guy that it's hard to find him even remotely likable. He seems barely capable of being nice to people, and I guess there is something interesting about watching a character like that eventually break down and let someone in — which he is going to do with Johnson, we are all assuming. But week-in, week-out, Masters is a frustratingly cold, standoffish fellow. He's not much fun to spend time with, which is why I'm grateful to have Caplan, FitzGerald, Ann Dowd, and Annaleigh Ashford, among others, around to both lighten and enrich the mood.

Though its ostensible lead may be a bit of a grating bore, Masters of Sex is quickly developing into a smart, arresting series, one that's far wiser about people than it at first seemed to be. That's a lovely surprise, especially from a network like Showtime, which can be quite adept at just-north-of-sleazy gunk, but falters when it goes for true drama. (Look at Homeland's frequent descents into silliness.) The show is still shallower than some of the truly great dramas on television, but there are glimmers of insight and, most excitingly, ambition, that bode well for the future of the series. In the parlance of the show, you might say it's close to arriving.