By casting Lizzy Caplan as Virginia Johnson, the creators of Showtime’s “Masters of Sex” did very well for themselves. Part of the show’s conceit is that William Masters (Michael Sheen) is charm-free, so the creators needed to find a co-lead who could act as a foil. As Emily Nussbaum wrote in her review, Caplan is “the swizzle stick in the show’s erotic cocktail.” She is the kind of performer who is so likable that you’d watch her tie her shoelaces for an hour, and she seems, in this show at least, to know that. She glides through her scenes with the poised self-assurance of a woman who has a strong handle on herself and her power.

I am not the only one watching “Masters of Sex” who wonders if things could have been the same for the real-life woman whom Caplan is playing. Post-“sexual revolution,” there is general agreement that a woman who knows what she wants, in bed and in life, is a person to be admired. But it’s much harder to believe that a woman in St. Louis, Missouri, in the late nineteen-fifties, could enjoy the same nods of approval from her contemporary onlookers. And by her own account, the real Virginia Johnson did not live the life relatively free of judgment and social cost that “Masters of Sex” has, so far, implied for her doppelgänger. (Warning: spoilers begin.)

Johnson died earlier this year, at the age of eighty-eight. She was no longer going by the name she’d used professionally for her forty-odd years in sex research. Instead, she was “Mary Masters,” another old woman in a nursing home with a story that only a few people listened to. Her tale is laced with regrets. As she’d tell the writer Thomas Maier, whose book “Masters of Sex” the series is based on, “I can remember saying out loud—and I’m appalled as I remember it—being very pleased that I could be anything any man wanted me to be. … In retrospect, I ask myself, ‘Geez, did I lose myself that totally?’”

Some of her guilt was the ordinary kind, familiar to any working woman. She worried that she’d missed her kids growing up. She was so busy working as Masters’s associate on his sex research that she never got the imprimatur of a university degree, an honorific that might seem ceremonial in retrospect but which meant a great deal to her personally. Most relevant, to those who have been watching the show, is how Mary seemed to regret her involvement with Bill Masters.

Masters and Johnson married only in the late nineteen-seventies. But long before that, they were lovers, as the show addresses, though lovers as a matter of clinical investigation. At least, that was how he described it. The television show more or less mirrors Johnson’s account of his initial proposition, in the sense that Masters wrapped it up in clinical language about transference and scientific precision. And yet, even in the nineteen-fifties, where we must rewind to some forgotten collective frame of mind before the sexual-harassment laws of the nineteen-seventies, the proposition still gives off a sour smell. A colleague of theirs speculated that had Johnson refused the proposition, she would eventually have been sidelined from Masters’s study. Confronted with his observation by Maier, Johnson seemed to agree. She told him, “Bill did it all—I didn’t want him… I had a job and I wanted it.” No one “forced” her to agree to the arrangement, but it was one made within a matrix of consequences that few think acceptable any longer. Not even in the context of sex researchers would we think it fair that having sex with your boss be an implicit condition of employment. In “Masters of Sex,” however, this quite serious situation is treated as a half-joke.

It is a curious choice not simply because it goes against our modern views but because it actually flattens dramatic possibilities. “Mad Men,” the show that “Masters of Sex” owes a strong aesthetic and tonal debt, has addressed similar themes. But somehow, “Mad Men” has managed to explain, implicitly, that its intelligent, even cunning female characters operated within sexual politics whose rules were not arranged for their benefit. Even when Bobbie Barrett told Peggy Olson, “You can’t be a man. Be a woman. It’s a powerful business, when done correctly,” the viewer was invited to raise an eyebrow. It’s not clear, when Bobbie leaves the picture, that she has played her hand correctly. It’s being comfortable with ambivalence that has always made “Mad Men” seem more grown-up than your average prestige-cable show.

By contrast, the Johnson of this show never makes a misstep, never seems seized by either regret or indecision. She becomes more like what the culture seems to want from “liberation”—a woman who has left the judgment of others behind, who doesn’t give a damn, who in her new omniscience sees both past and future clearly—and less like any recognizable human being who has ever lived. For all the bitter quotations that the real Johnson gives in Maier’s book—friends say, in fact, that the “hard things” Johnson had to say about Masters ultimately drove them from her company—they make up a more rounded personality than the one “Masters of Sex” presents.

There is one final spoiler that further complicates the liberation: Masters eventually left Johnson. Their partnership of many years, their professional fulfillment together, was less important to Masters than the fulfillment of a fantasy. He asked Johnson for a divorce on Christmas Eve in 1992, and the proximate cause was his reconnection with a woman he’d tried to propose to some fifty-five years before. He’d later marry that woman, and remain married to her until he died, in 2001. Johnson would remain alone. She would still say she had never loved Masters. But she didn’t like being single either. “I like being married—I hate not being married now,” she told Maier. As it turns out, not every kind of freedom is the one you’d pick out for yourself.

Photograph by Peter Iovino/Showtime.