On a summer night in 2011, James Hodgkinson, a sweet-faced, 28-year-old British paramedic trainee, was knocked to the ground by a stranger’s single, unprovoked punch to the head. Certain facts about that night and its terrible aftermath are burned into his parents’ memories like the tenderest of scars.

His father, David Hodgkinson, was with him outside a pub in Nottingham, England, when it happened, the two of them visiting from out of town. James’s mother, Joan Scourfield, got the news and raced north to Nottingham, where he was hospitalized.

“I threw stuff in a holdall, no clothes, just random stuff,” she said in an interview in Manhattan earlier this month. “You know, not thinking at all. Your panic is to get there, isn’t it? And I wasn’t thinking I’d be there, I guess, that long.”

She wasn’t thinking, either, that her healthy, adventuresome son was in mortal danger from one punch. But nine days after a local 19-year-old spoiling for a fight aimed his fist at James, she and David made the excruciating decision to have him taken off life support. Jacob Dunne, the young man who killed him, served just 14 months for manslaughter. The brevity of his sentence, which they unsuccessfully appealed, still nettles them.

So it is utterly counterintuitive that Scourfield and Hodgkinson are now on warm, collaborative terms with Dunne, and that he has often credited them with saving his own life — by asking, after he got out of prison, what he planned to do with it, and genuinely caring what he answered. Hodgkinson, who saw the punch and relived it in his nightmares for months after James’s death, said he has come to feel “quite protective” of Dunne.

How they all arrived at that point is the story of “Punch,” a new play by the British playwright James Graham (“Dear England,” “Ink”), who grew up near Nottingham. Adapted from Dunne’s memoir, “Right From Wrong” — and, as such things often go, lightly fictionalized in the transformation — “Punch” is running simultaneously on Broadway and in London’s West End, with Jacob, Joan and David the drama’s principal characters.