Bill Masters and Virginia Johnson spend much of the second season premiere of the Showtime drama "Masters of Sex" (Sunday at 10) trying to redefine the nature and parameters of their relationship after the events of the show's marvelous first season. Are they now a couple? Is this just an affair? Is it still simply one small component of their groundbreaking study on human sexuality?

If Masters (Michael Sheen) and Johnson (Lizzy Caplan) need more time to figure out what they're doing, "Masters of Sex" certainly does not. It was one of TV's very best dramas a year ago, and in many ways the start of season 2 suggests it's gotten even better.

The first season ended with both disaster and epiphany, as an early version of the famous Masters and Johnson study was greeted with horrified indignation by Masters' medical colleagues, but also with the suddenly-unemployed Dr. Masters finally admitting that his desire to sleep with Mrs. Johnson had little to do with contributing to the study and a whole lot to do with his emotional feelings for his adventurous, independent, sexy, whip-smart partner.

(Warning: the next paragraph will fit the Matthew Weiner definition of a spoiler, and only the Matthew Weiner definition of a spoiler.)

This was a big moment. Given that "Masters" creator Michelle Ashford has a whole lot of chronological ground to cover just to get from the late '50s of season 1 to the 1966 publication of Masters and Johnson's book, she could have easily pulled a "Mad Men" and bounced forward significantly in time, only revealing later what happened in the immediate aftermath of Master's declaration to Johnson. But the new season only jumps ahead a small amount — enough time for various characters' situations to have advanced, but not so much that we miss all of the real emotional tumult from the events of last year.

So we get to see just how complicated things remain between Bill and Virginia — he did, among other things, run to tell his feelings to Virginia on the day his wife Libby (Caitlin FitzGerald) gave birth to his son — and we also get to see Bill's mentor Barton Scully (Beau Bridges) continue his primitive gay conversion therapy for the sake of his wonderful but unhappy wife Margaret (Allison Janney). Both Bridges and Janney were amazing in the first season (and were, like Lizzy Caplan, deservedly nominated for Emmys yesterday), and though their availability this season is limited due to their CBS sitcom commitments, Ashford and company get outstanding use out of them when they're around.

There is, in fact, a fair amount of turnover in the supporting cast. Annaleigh Ashford (no relation to Michelle), who plays ex-prostitute (and sometime expert source for Bill Masters) Betty, had to leave the first season early to star in Broadway's "Kinky Boots." Now she's back — and Betty plays a crucial role in the next phase of Masters' work — while Bridges, Janney, Nicholas D'Agosto (Virginia's boyfriend Dr. Haas), Rose McIver (Barton and Margaret's daughter) and Heléne Yorke (frequent sex study participant Jane) are all moving on to other gigs, while still popping up when they can. But there are many impressive new faces, including Danny Huston and Betsy Brandt, plus the reassuring return of others like Teddy Sears (as cheerfully adulterous Dr. Austin Langham) and Julianne Nicholson (uptight, cancer-stricken pap smear advocate Dr. Lillian DePaul.)

The main attraction, though, remains Masters and Johnson, and Ashford has somehow found a way to even more intensely focus the show on these two temperamental opposites, their dysfunctional relationship, and their common fascination with both sex and each other. These remain wonderful performances from Sheen and Caplan, and the writers do an impressive balancing act of shifting sympathies between them — especially considering that Masters is such a cold, controlling bastard much of the time. Virginia is the warmer, more open, more inherently likable of the two, yet there are ways in which she is weak, or selfish, or just wrong, and times when Bill can be remarkably compassionate, insightful and/or articulate about his needs or the needs of others.

It's a great duet between actors, and characters, and the relationship is so full of twists and turns and ambiguity that the show could probably get away with eliminating all the other characters and just present a series of one-act plays featuring only Masters and Johnson.

In fact, that's essentially what the season's terrific third episode, "Fight," is. Without giving too much away, what's remarkable about the episode is how it simultaneously feels exactly like the famous "Mad Men" episode "The Suitcase" (much of the action even revolves around a heavyweight title fight), and also really nothing like it, because of the specificity of these two characters and their relationship.

Now, I wouldn't want to eliminate all the other fascinating characters who walk in and out of Bill and Virginia's life. They're played by great actors in their own right, and those other stories shed copious light on the ways in which sexual and emotional issues have changed enormously since the '50s, and the ways in which they haven't. "Masters of Sex" has much more on its mind than simply the tumultuous relationship between its two famous central characters. But if it just had those two, it would still be among the best things you could watch on television this summer.

Alan Sepinwall may be reached at sepinwall@hitfix.com