As "Masters of Sex" heads into its fourth season, the Sexual Revolution is in full swing -- but the sex researchers who fired the opening shots are learning they may not quite be ready for the upheaval they initiated.

"We start this season with Virginia Johnson sitting in a hotel room watching this protest in the Miss America pageant in 1968 where women are just outraged and traipsing along the Boardwalk in Atlantic City with a sheep with a crown on its head and throwing their bras into trash cans," executive producer Michelle Ashford tells Moviefone of the Showtime series' game-changing new season, which launches on September 11th on Showtime.

"It's such a curious moment because she was always such a radical really in many ways, and now the world has not only caught up, but surpassed her because she likes her bra. We really thought, 'Wow, this is a really energizing time!' Every character is undergoing great, great transformation."And no one feels the transformative vibes as deeply as Bill Masters, according to the show's leading man Michael Sheen. "In terms of our story, we're at the midpoint really, and I always wanted him to be in the most troubled and the most restricted place," the actor explains. "He's being accused of pedophilia; his marriage has fallen apart; he's admitted what's going on -- he's literally in prison. And yet, in terms of his own personal life, he's the most free he's ever been. He's not living a lie anymore. He's starting to become his more authentic self. I like that kind of juxtaposition."

"In this season, we see a man who is really at a crossroads in his life," adds Sheen. "Things are going to change again even more. He's on his own now. We don't even know if he's going to carry on with the practice or not. Obviously, if you know the facts of what happened in their life, you know certain things that are going to happen, but the interesting thing is: how is it going to happen? In what way are we going to get there?"

"Now he's at the tipping point," agrees Ashford. "He's tipped over into something else, and he is now trying to figure out who he is in a world that is exploding around him, and with disastrous relationships, and he has to sort of start at square one ... He realizes he's a very different man than the man he's been all these years. That actually felt like a weird persona he had adopted, and it doesn't feel right anymore, and hasn't served him well."Playing Virginia Johnson, Lizzy Caplan says the evolution of the characters -- both in terms of the historical realities and their somewhat more loosely interpreted TV alter egos -- has been fascinating to have a hand in.

"They're starting to change in very dramatic ways in this season," says Caplan. "One of the main things we wanted to keep our eye on when we started the show is, that when you first meet Bill and Virginia, Bill is this buttoned-up, cold, clinical man, and Virginia is this ray of light and this sparkle of a human. And towards the end of their time together -- we're not there yet, but after the 30-year time period is over -- they sort of switch roles, and she hardens quite a bit as Bill softens and becomes more personable. We're starting to get into that in this season."

"When you meet her again, she's a mess, drinking quite heavily, up to all kinds of self-destructive behavior," says Caplan, "because she is without her center, which is her work that she left last season. We didn't know if she was going to come back or not. And without her work, she's not a whole human being, so it's her way of finding herself to her work, which in her mind, is very much equated with Bill."

As she contends with her ambivalent relationship with the change in sexual mores she helped spark and her increasing status as a role model and potential mentor for a generation of women coming up in the path she cut through the socio-sexual wilderness, Caplan says Virginia nevertheless explores even more then-taboo territory. "Their second book is 'Human Sexual Inadequacy,' and so they're in the middle of trying to get that published," she says, "and then they're starting to study deviancy and then homosexuality, which then leads to some of their dark days, which are still to come."

The transformative spirit spreads to the show's other regular characters as well, promises Ashford. "The crazy one is Libby -- our actress [Caitlin FitzGerald] thought she was going to be fired once the Masters got divorced!" the producer chuckles. "Quite the opposite has happened, because she has the most remarkable journey of all. I love it because she started as the most repressed. As happened with many women of that generation, in getting divorced, in having her life implode, actually a new path of freedom and openness emerged before her, and she takes it."

"We have a crazy story coming up where Libby and her new beau go on a trip and they get stuck in a traffic jam, and you don't realize until a great deal through the episode that the traffic jam is going to Woodstock," teases Ashford. "And by the way, Woodstock was not Woodstock until after the fact, so for them, it's a traffic jam. So hopefully that's how we come at history and bring it in."

Another historical aspect to be dramatized is the role Playboypublisher Hugh Hefner, himself a major provocateur in the Sexual Revolution, played in the lives of Masters and Johnson."We have history to back us up on this one: they were very close, Masters, Johnson, and Hefner, and they spent a lot of time at the Chicago Playboy Mansion," says Ashford. "Ironically, they would take separate bedrooms to keep up appearances, which of all the places in the world you wouldn't have to keep up appearances, it would have been that place!"

"The thing is, despite the magazine and the centerfolds and all that, he was a very serious man about many things," reveals Ashford, "so he really connected with them on the fact that they were very serious about the study of sex, and by demystifying it. They had a very long relationship. He's such a great character because, of course, you get all the girls on roller skates and all that stuff, but they were very serious together about the work they were doing."Some new faces are added into the mix as well, with actors Nurse Jackie") as Nancy, a former medical student of Barton Sculley's (Beau Bridges), and Jeremy Strong ("The Big Short") as Art, a Kinsey Institute psychologist, joining the cast. "Once Masters and Johnson's clinic kept growing, and their fame had grown, and they were very busy, and they were touring and doing all that stuff, they brought more doctors in to work with them," says Ashford. "So we bring in a new psychiatrist and a new M.D.

"Masters and Johnson are needing to keep a distance because of their sort of fractured relationship, paired with the new people: he with a woman, and she with a new man. They very quickly realize, 'No, no -- actually, they are much better working together,'" says Ashford. "But these new characters have turned out, in the way that Allison Janney and Beau Bridges's characters had their own lives, to become very central to our series this year. They really, really shake things up."

As the timeline segues into the even more wildly experimental '70s, the show will continue to explore the same envelopes society was pushing. "We have an episode that's set at a nudist colony," promises Ashford. "We had 160 actual nudists who came and were extras. People would say, "When are we going to see a man naked? When are we going see women totally naked?" So we said, "Okay, you want that? Here's 160 of them -- there's more penises in that episode than you'll ever see on TV, ever." We were just having a ball. Everything is now just loose and you never know what's around the corner -- Bill Masters is smoking pot! It's just all up there."