The third season of “Masters of Sex” starts slowly, missing absent supporting characters and absorbing a significant time lapse that feels a trifle disorienting. Things do pick up, but with one slightly discordant note as the series increases its focus on the central characters’ now-teenage children, while running an onscreen disclaimer that says Masters & Johnson’s progeny as depicted are wholly fictional — raising the question, “Then why bother?” If that risks confusing the fact-based underpinnings, the series generally compensates, once again, with fine performances from the trio of Michael Sheen, Lizzy Caplan and Caitlin FitzGerald.

The program has jumped to 1966, the eve of William Masters and Virginia Johnson (Sheen and Caplan, respectively) publishing their breakthrough book, with the fame and notoriety that unleashed. In the premiere, they’re vacationing with Masters’ wife (FitzGerald) and both the Masters’ and Johnson’s kids, reflecting what amounts to the three-way marriage into which the protagonists have settled, happily or otherwise.

Venturing further into the ’60s brings with it other real-world intrusions, including the war in Vietnam and the sexual revolution. Prickly in the best of times, Masters is especially agitated by the prospect of having to defend his work to the press, while Virginia continues to chafe at those who would question her contributions and credentials.

Although several high-profile guest stars are due later in the season, the opening two hours focus so intently on the principals and family affairs that the lack of familiar supporting players feels more acute. In addition, emphasizing the kids — while clearly a device to enhance insight into their parents, especially Masters given his horrible relationship with his own father — feels undermined by the lawyerly language ostensibly tacked on to prevent casting anyone still living in an unflattering light.

Dramatically speaking, it’s easy to see why the producers would explore these plot threads. Cable dramas, however, take such detours at their peril — something Showtime certainly learned the hard way when “Homeland” devoted a good deal of attention to the teenage daughter. (As a footnote, it’s worth citing that while the children are markedly older, other than hairstyles, nothing much has been done to indicate the toll of time’s passage on the leads.)

Thankfully, the opening hour closes with another riveting exchange involving Sheen and Caplan, whose quiet moments together have been consistent highlights. The second episode further complicates their partnership, fueled in part by the tensions associated with unveiling their work. Less successfully, a special patient seeks Masters’ fertility help, which, frankly, feels a bit too on the nose in terms of creating a parallel to the thornier aspects of the triangle in which the good doctor finds himself.

Despite those creative speed bumps, “Masters of Sex” remains a slick vehicle, one that thoughtfully examines the relationship between sex and romance that Masters and Johnson sought to uncoil — far less effectively, it’s worth noting, than either of them would care to admit.

Still, just as “Mad Men” hit rough patches as it advanced chronologically from the Eisenhower era into the counterculture movement, these initial hours flash a few warning signs as well. While the leads provide ample incentive to keep watching, “Masters Of Sex” will maintain its status as one of the true Masters of its dramatic domain only if the series finds a way — given the narrative hurdles it’s erected — to get over those humps.

TV Review: ‘Masters of Sex,’ Season 3

(Series; Showtime, Sun. July 12, 10 p.m.)

  • Production: Filmed in New York by Round Two Prods. and Timberman/Beverly Prods. in association with Sony Pictures Television.
  • Crew: Executive <a href="http://variety411.com/us/los-angeles/producers/">producers</a>, Michelle Ashford, Sarah Timberman, Carl Beverly, Amy Lippman, David Flebotte, Judith Verno; supervising producer, Esta Spalding; producers, Thomas Maier, Tammy Rosen, Michael Sheen, Matthew Carlisle; director, Jeremy Webb; writer, Ashford; based on the book by Maier; <a href="http://variety411.com/us/new-york/camera-sound-equipment/">camera</a>, Christopher Manley; production designer, Elizabeth H. Gray; editor, Blake Maniquis; music, Michael Penn; casting, Risa Bramon Garcia, Bernard Telsey, Jami Rudofsky. <strong>60 MIN.</strong>
  • Cast: Michael Sheen, Lizzy Caplan, Caitlin FitzGerald, Annaleigh Ashford
  • Music By: