“Downton Abbey” fans may be savoring their final hours on their favorite English country estate. But in a nice piece of timing, this season’s big “Downton” plot, a fight over a proposed hospital takeover, may be preparing “Downton” fans for a new show PBS is launching at 10 p.m. Sunday night. “Mercy Street,” a drama set in a military hospital in the early years of the Civil War, aims to combine PBS’s standards for period fidelity, a potent cocktail of changing social mores and a conflict that would come to define the nation, and a little bit of smoldering sexual chemistry for good measure.

If “Downton Abbey” drew its drama from the spectacle of highly traditional characters confronting the inexorable changes advancing across England, “Mercy Street” focuses on a set of characters who are already in transition.

Mary Phinney (Mary Elizabeth Winstead) is a widow who is assigned to Mansion House Hospital by Superintendent of Army Nurses Dorothea Dix (Cherry Jones), who had been given the authority the appoint female nurses over the objections of doctors and surgeons. Samuel Diggs (McKinley Belcher III) is a free black man who grew up in a physician’s home and who tries to make use of his extensive medical knowledge without overstepping the expectations of what a black man can or can’t do. Emma Green (Hannah James), the daughter of a privileged Confederate family whose hotel has been requisitioned for the hospital, defies the expectation for her gender and class by volunteering as a nurse. And Dr. Jedediah Foster (“How I Met Your Mother” star Josh Radnor), an experimental surgeon from a slave-holding family in Maryland, is trying to battle and conceal a morphine addiction.

“He’s this progressive, forward-thinking doctor who’s studied in Europe and is on the cutting edge of knowledge about different advances in medicine, and at the same time grew up on a Maryland plantation with slaves, and he’s certainly not evolved when it comes to abolition and the issue of slavery, which is a central conflict with Mary’s character,” Radnor told me when we spoke about his experience playing Foster in December. “What really floored me is that being a doctor wasn’t a prestigious profession back then. The bar of entry was so low. . . . You would apprentice with your friend, the country doctor, for a couple of months, and he’s like ‘Congratulations, you’re a doctor!’ It wasn’t the way it is now. So he takes it very seriously, but his mother thinks he’s betrayed the family by not taking over the estate.”

For Winstead, part of what was exciting about playing Phinney was the opportunity to portray a character whose views are commonplace today but were considered radical in their time.

“I almost felt like I had it easy, because women like Mary Phinney and Louisa May Alcott [both of whose nursing experiences informed Winstead’s character] and women of that time had these personalities that were so modern, and so like they were plucked and sent back in time and landed in an era that was unfamiliar to them,” Winstead told me last fall. “They’re sort of like ‘Everyone is so crazy, there are these basic truths.’ They believed that so passionately that it feels like a modern sensibility, so so much of the dialogue and how the characters sort of behave — or my character specifically — felt very relevant to now for me.”

These moments of transition produce fierce clashes over status and theories of medicine. Phinney, trained by Dix, finds herself in conflict with Anne Hastings (Tara Summers), who worked with Florence Nightingale in the Crimean War, when Dix appoints Phinney to be the head nurse at Mansion House Hospital.

“The number of dead, the number of wounded, you would think they would want to take help from anyone who would give it,” Winstead said of how hard women had to fight to be allowed to serve. “But to think that women were still kind of turned away, or treated with disdain, or treated like they couldn’t be helpful; the women who actually did gain some respect and gained a place, you can see how they would be threatened at the thought of losing that.”

“The Civil War truly was a time when women, for the lack of a better word, came into the workplace,” PBS chief programming executive Beth Hoppe told me when we spoke about the show in September. “It was also a time when medicine was undergoing huge changes. The doctors were experimenting on these patients, but they were truly changing the way everything was done.”

And Winstead said that part of what she appreciated about “Mercy Street” was getting a part in a project with multiple female characters — and where women find each other in conflict over power and intellectual traditions, rather than simply for the sake of drama.

“I think most actresses would say we’re used to reading scripts where we’re the one woman in a cast of 10 guys. And being that one woman, you’re probably someone’s girlfriend or someone’s wife, so you have one relationship and you have no life outside of that relationship, whatever story point you’re at. So that’s always kind of the norm, and it’s always incredibly boring,” Winstead said of many of the scripts on offer. “I literally, from the first two pages, was like, I have to do this. And the fact that it was directed and produced by women, knowing I was going to be going into something that was from a really strong female point of view, [playing] this really interesting woman, and working with women behind the scenes to create it was just one of those things, there’s no way I’m not doing this. This is important for me.”

Radnor observed that his character, who did his medical training in Europe, finds himself in conflict with Dr. Byron Hale (Norbert Leo Butz) for similar reasons: “Those are just status arguments. And with Mary [Phinney], especially at the beginning, those are status scenes, always. The whole Mansion House Hospital is kind of a petri dish of different statuses,” in the context of a war over “who is even considered to be a human and what rights are conferred upon people.”

“Mercy Street” came together quickly: both Radnor and Winstead found out that they had won their parts just a few days before they had to report to Richmond, Va., where the show was shot.

Radnor prepared by focusing his research on three different tracks: learning about the Civil War, getting briefed on period medicine (“Mercy Street” shares a period medical consultant, Dr. Stanley Burns, with “The Knick”), and studying addiction, including by reading Thomas De Quincey’s “Confessions of an English Opium Eater” and interviewing a friend who is a recovering heroin addict about “what it felt like and what he thought he was getting from it and how it turned on him.” Winstead read Phinney’s biography and Alcott’s “Hospital Sketches.” And the fast pace and long days of the shoot, which required Winstead to wear heavy dresses and corsets while doing physical labor, helped her channel what she imagined must have been Phinney’s constant exhaustion and dislocation.

But despite the speed of the project, Hoppe said the “Mercy Street” cast  “came together as an ensemble in a way I haven’t seen. . . . It’s this amazing golden age of drama. People are saying there’s too much, but I don’t believe there can be too much, because there can only be a few things that are head and shoulders above the rest. Josh Radnor has said this is a turning point in his career, and that’s exciting. We want to provide that.”

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