Though Jedediah’s mother is irate that her son has abandoned the family plantation to study medicine, she wants him to save his brother’s life and leg after he is gravely wounded. And when that proves impossible, Foster must perform an amputation for the first time with the help of abolitionist nurse Mary Phinney (Mary Elizabeth Winstead) and Confederate volunteer Emma Green (Hannah James), whose family hotel has been seized for the hospital. The resulting scene is grisly and medically accurate, but it’s also an intense three-hander that forges new relationships between the show’s main characters.

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For PBS, calibrating the bloodiness of the scene, and “Mercy Street” as a whole, was a challenge.

“There were places where I was like, ‘I get it, guys, but do you have a wide shot so we don’t have to see that?’ And ‘How many seconds can we stay on that close-up of that thing?’ ” Beth Hoppe, PBS’s chief programming executive, told me. “What was easy was that we were going to go there. I love ‘Downtown Abbey’ beyond the beyond, but if you think back to the episode where Sybil gave birth and died, there was not a speck of blood and everything was pristine and clean. … At its core, it was always a lighter piece than this is. We’re premiering it at 10 on Sundays; it’s going to repeat at Mondays at 9. We’re not putting it at 8 and there’s a reason for that.”

Winstead said that the scene was very early in the shooting schedule. Stanley Burns, the medical consultant for “Mercy Street” as well as “The Knick,” helped all three actors learn their specific tasks — for Winstead, administering ether, applying tourniquets and holding arteries — and Winstead, Radnor and James worked together to block out the scene. But on the day in question, they had to get along on adrenaline.

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“It was really disgusting and incredibly realistic. We were shooting in such a fast way that it felt very real. My heart was just sort of pounding shooting it,” Winstead said of letting the momentum of the scene carry her along.

Radnor agreed that while it initially seemed daunting, the timing of the scene proved fortuitous.

“I went, oh, wow, okay, I have to cut off my brother’s leg. I’ve certainly never done anything like this before as an actor,” he said. “My instinct was that they scheduled it wrong, this is going to be terrible. But the character was supposed to be overwhelmed and in over his head and terrified, and as an actor I felt those things anyway. So when I watched the scene, I remembered feeling in over my head, what am I doing, it’s crazy that I’m throwing myself into this. But I saw that the scheduling gods had actually done me a favor because it was a moment for that character that was beyond terrifying and overwhelming, and it actually kind of worked to my advantage.”

And the timing also emphasized the limits of medicine during the Civil War.