WASHINGTON — An icy January afternoon was turning into evening, and inside a warmly lit rehearsal room at Arena Stage, the company of a new play called Sovereignty had arrived at the final scene.

The sweet, 21st-century ending unfolds in an unlikely setting: a family cemetery in rural Oklahoma, not far from the spot where, in 1839, a Cherokee Nation leader named John Ridge was stabbed to death in an act of political retribution. His influential father, Major Ridge, was assassinated the same day, and for the same reason.

The playwright, Mary Kathryn Nagle, is one of their direct descendants on her father’s side, and in Sovereignty she is exhuming some family history that is also American history. Both John and Major Ridge were signers of the bitterly divisive treaty — vehemently opposed by the Cherokee chief and many others — that removed the tribe from its land in the Southeast and sent thousands on the Trail of Tears to Indian Territory, in what is now Oklahoma. The Ridges’ killers were fellow Cherokees, wreaking vengeance.

“It’s my family onstage,” Ms. Nagle, 34, said after rehearsal, in a greenroom in the chilly bowels of the theater. “It’s the story that was told to me from the time I was this big. I’ve carried it in me my whole life.”

What she has made of that story, a time-shifting play whose characters include President Andrew Jackson, is in keeping with Arena’s penchant for political fare like John Strand’s The Originalist, about the Supreme Court Justice Antonin Scalia, and Lawrence Wright’s Camp David.

Ms. Nagle — a lawyer who wrote and, with her fellow students, staged a play each year she was at Tulane Law School, and met her law partner when he came to the Newseum to see a reading of another of her plays — fits right in, not only with Arena but with Washington.

“She has an ability to quickly move from the personal to the political,” Molly Smith, Arena’s artistic director, said. “We live here, where we eat, sleep, drink politics, and it’s all through our personal lives. She embodies that within the work that she does.”

Opening on Jan. 24 in a world-premiere production directed by Ms. Smith as part of the Women’s Voices Theater Festival, Sovereignty came out of Arena’s Power Plays initiative, which aims to tell a story of the United States in 25 new works over 10 years, with one play pegged to each decade since 1776. Ms. Nagle is the first Native American voice in that mix.

“I started writing snidbits of this play in law school,” Ms. Nagle said, and in that casual, playful little word “snidbits” is a counterbalance to her cerebral intensity, the ability she has to cite case law and obscure dates mid-conversation. (She is also highly entertaining, given to talking with her hands and throwing her arms wide to emphasize a point.)

Ms. Nagle radiates the energy that her résumé alone suggests: a full-time law career, devoted to the issues that also consume her writing (tribal sovereignty, the environment, domestic violence and sexual assault); two plays getting world premieres this year on opposite sides of the country, the other being Manahatta at Oregon Shakespeare Festival; a part-time gig running the Yale Indigenous Performing Arts Program in New Haven.

She moved to Washington in 2015 after a stint working for a corporate law firm in New York, where she wrote Manahatta in the Public Theater’s Emerging Writers Group. Last summer, she moved back to Oklahoma, where she grew up. A glow came into her face when she mentioned her new house, on a lake on the Osage Reservation, but her Tulsa-based firm has an office and an apartment in Washington, and her travel-heavy practice still brings her there. When I told Ms. Nagle that I wondered not about her work-life balance but simply how she juggles her legal career with her art, she leapt right into the personal side anyway.

“I don’t have children, and I would like to have children,” she said. With an upbeat, what-the-hell forthrightness, she added, “Let me just put that advertisement out there for who wants to be the stay-at-home dad, ’cause I don’t think I can handle a third thing right now. It’s kind of overwhelming.”

The daughter of a doctor and a nursing school dean, Ms. Nagle started making up stories as a child, dragooning her two younger sisters into acting them out with her. As a Georgetown University undergraduate, she designed her own major in justice and peace studies, but took classes in theater, won a student one-act contest and wrote a play called Miss Lead, about lead mining on Oklahoma reservations, for her senior thesis.

Her freshman year, she performed in a student production of Paula Vogel’s domestic violence play Hot ’n’ Throbbing — a formative experience that Ms. Nagle said shattered her impression that there were “certain things we experience as women that are not appropriate for the stage.”

When Ms. Smith asked her in 2015 what she might like to write about for Arena, Ms. Nagle immediately thought of the Violence Against Women Act, which was strengthened in 2013, giving tribal courts the power to prosecute non-Native Americans who victimize Native American women on tribal land. Present at the signing ceremony, watching President Obama make that change into law, Ms. Nagle sobbed.

She is convinced, though, that there will eventually be a challenge to that protection — “when a non-Indian tries to argue to the Supreme Court, which they will, that any exercise of tribal criminal jurisdiction over a non-Indian is unconstitutional.”

As a lawyer, she is preparing for that scenario. As a playwright, she is imagining it in Sovereignty, a drama about broken treaties and historical rifts that is also about rape.

It was Ms. Smith’s idea to broaden Ms. Nagle’s original concept, interweaving the contemporary strand — centered on a Cherokee Nation lawyer who strongly resembles Ms. Nagle and becomes involved in a domestic violence case — with one about her Ridge ancestors. Before Major and John Ridge signed the treaty to hand over tribal land, they were instrumental in a rare case where Native Americans prevailed in the Supreme Court: Worcester v. Georgia, in 1832, establishing a crucial precedent about tribal sovereignty.

That victory is the proud story Ms. Nagle was raised on, the story her grandmother told her that gave her faith in the Supreme Court and made her want to go to law school. The play follows them through that case and the treaty signing to their deaths.

The cemetery, by the way, the one where the Ridges are buried: That’s where Ms. Nagle plans to end up, too, one day.