Masters of Sex's foreplay is over. Season 3 is the payoff.

When the Showtime series returns (Sunday, 10/9c), it's 1966, the year when Dr. William Masters and Virginia Johnson finally publish the results of their research in their seminal book Human Sexual Response, and neither is ready for the fame and other consequences that will follow. But that's what you get when you're partially responsible for the sexual revolution.

Here a look at what's to expect this season:

Dr. William Masters (Michael Sheen) As usual, the good doctor isn't as cool-headed and objective as he'd like to believe, and he'll soon regret certain perfectly logical decisions he's made, especially regarding his research partner. Nevertheless, the notoriety from the book's success will challenge Bill with new, sometimes high-profile clientele and open the door for new research opportunities. At home, his wife has become more distant, and Bill's fears about repeating his own father's parenting mistakes will be tested.

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Virginia Johnson (Lizzy Caplan) Virginia will take big, life-changing steps to make sure that nothing about her background or lack of credentials will undermine their work. Also, although she and Bill are still going strong, more than one man will tempt her to stray from him. Meanwhile, being a good mother becomes one of her chief concerns when both of her teenage kids start to rebel against her rather lax, and more often absent, parenting. Guest stars Frances Fisher and Michael O'Keefe will also appear this year as Ginny's disapproving parents.

Libby Masters (Caitlin Fitzgerald) When we last left the poor hausfrau, Libby had finally accepted her husband's infidelity and decided to have her own affair. This season, she'll attempt to make a pact with Virginia to solidify their odd, yet amicable husband-sharing relationship, but is again left floundering when it comes to finding her own happiness. As she's having one of those "grass is greener" pity parties, a man will come into her life.

Betty DiMello (Annaleigh Ashford) Although Betty wasn't able to conceive, she hasn't given up the baby dream yet. The love of her life, Helen (Sarah Silverman), is back, and they want to have a child together. As for the father, he won't be a stranger to fans of the show.

Margaret Scully (Allison Janney) Having been married to a gay man, Margaret is now making up for lost time with a certain sexy scientist played by Tate Donovan. The two end up in Masters and Johnson's clinic, however, so maybe they're not entirely successful.

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Daniel Logan (Josh Charles) The Good Wife alum plays a business-savvy scent peddler who wants to bottle the smell of sex as an aphrodisiac. When he consults Masters and Johnson, he becomes a bit more involved than he had planned.

Dr. Christine Wesh (Maggie Grace) This incredibly well-educated and competent gynecologist has all the qualifications that Virginia lacks. Will that be enough for Bill though? When an internationally charged case comes in, Dr. Wesh is on hand to offer her expert opinion.

Nora (Emily Kinney) The Walking Dead alum plays a blast from the past for Bill, who once hired the young neighbor girl as a babysitter. Now she's all grown up (sort of) and wants to participate in the clinic's research as a sex surrogate.

Masters of Sex returns on Sunday at 10/9c on Showtime.

(Full disclosure: TVGuide.com is owned by CBS, Showtime's parent company.)

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