By the time she was twenty, Marin had relocated to Tampere, a post-industrial city once known as the “Manchester of the North.” She was living with her boyfriend, Räikkönen, whom she had met at a bar called Emma. (“We totally forgot that was the name,” Marin told me. “We had no idea until recently we named our daughter after a bar.”) One day, Marin decided to attend a meeting of the S.D.P.’s youth organization. “When I walked into the room everyone just stared at me,” she writes. It was rare for a person without a social connection, someone just off the street, to get involved in Party affairs. She found the meeting underwhelming. The attendees were debating whether they should buy lunch for volunteers at an upcoming event. “I couldn’t believe it,” she writes. “These were young people, my peers. Weren’t we supposed to be the most passionate members of the political system? Where was the revolution?” And, she adds, lunch “should have [been] provided, without question or argument.”

Marin enrolled at the University of Tampere in 2007, and there she found her cohort. The school had a reputation as a “red campus.” (The filmmaker Aki Kaurismäki, who is known for his absurdist social-realist films, studied there in the late seventies.) Marin joined reading groups, where she read “all the socialist classics.” The following year, she launched her first campaign, a run for city council, which she lost. Her slogan was “Four Targets in Four Years.” I asked what the targets had been. “I don’t remember,” she said. “They weren’t that ambitious, something about recycling.”

The S.D.P., which Marin officially joined in 2007, was actually an odd choice for a Gramsci-reading freshman. Founded in 1899, it was increasingly viewed as out of touch, but she saw herself as part of a movement to revitalize the once storied workers’ party. The times required it. When the global recession hit Finland, the government implemented austerity policies that harked back to the days of eraser splitting. For millennials, who were now starting out in their adult lives, it was a galvanizing moment.

The Finnish media began inviting young up-and-coming political figures—including Marin, who in 2010 became vice-chair of the S.D.P.’s youth organization—to participate in televised debates. Another rising star was Li Andersson, who belonged to the youth organization of the Left Alliance, a party to the left of the S.D.P. “We were on this show—it translates very strangely—but it was called ‘Hate Evening,’ ” Andersson, who is now a member of the European Parliament, told me. She knew Marin only by reputation: “Sanna was seen as being more on the red-green side of the Social Democrats, so more modern.” (The term “red-green” in Finland describes people who support workers’ rights and environmentalism.)

Marin won a seat on the Tampere City Council in 2012, running as a left-leaning S.D.P. candidate. Using Photoshop, she had made her own posters, which she and Räikkönen passed out on the street. (“I have handed out tens of thousands of flyers,” Räikkönen told me.) Marin was appointed leader of the city council at twenty-seven, the youngest person ever to hold that position.

But Marin’s true star turn came in 2016, after a clip from an hours-long city-council meeting that she led went viral. Marin was trying to move along a vote on a green initiative: the construction of a three-hundred-million-euro tram system. It was a big price tag for Tampere, a city known for its shuttered textile factories. Several council members dragged out the proceedings, with one speculating that unemployed people might “ride around together on the tram as there is nothing else to do.” At the front of the room was Marin, then thirty, training her icy blue eyes on each person trying to stall. “Is Council Member Kaleva seriously asking for yet another turn? Last time, you were reading out a newspaper column.” Marin prevailed, and the video now has nine hundred thousand views, equivalent to a sixth of the population of Finland.

Around Christmas of 2018, just months before the general election, Antti Rinne, the leader of the S.D.P., fell ill and was reportedly placed in a medically induced coma. He recovered, and that June, after the S.D.P. won more seats in Parliament than any other party, Rinne became Prime Minister. But six months later he was forced to resign, after he was accused of mishandling a labor dispute at the expense of postal workers, drawing rebuke from the Center Party, whose support he needed to govern. His coalition fell apart, and, in an intra-party election held to succeed him, Marin won by three votes against a more centrist male challenger.

“Nobody in Finland was thinking about her age or gender,” Salla Vuorikoski, a journalist for Helsingin Sanomat, the country’s largest newspaper, and the author of a 2024 biography of Marin, told me. “We knew her as a minister from Tampere. But when she had that first press conference I turned to my husband and said, ‘This is going to be huge abroad.’ She looked different.”

Before arriving in Helsinki, I watched “First Five,” an HBO documentary series from 2023 about Marin and the other party leaders—Maria Ohisalo, Annika Saarikko, Anna-Maja Henriksson, and Andersson—in her government. “First Five,” which is mostly made up of sit-down interviews and news clips, felt a lot like a Finnair in-flight safety video: reassuring in terms of national welfare but a tad impersonal. (The most interesting tidbit is Andersson telling her friends that Bernie Sanders called to ask about parental leave and early-childhood education in Finland.) “I don’t even remember doing the documentary,” Marin told me. I wondered if the series was flat because its subjects had grown tired of talking about their “lipstick government,” as some critics had begun calling it. As Andersson said, “It was, like, ‘Oh, wow, they’re all making decisions together in the sauna.’ ”

The next day, Marin gave me a tour of Kesäranta, a villa in a leafy part of Helsinki that serves as the official residence of the Finnish Prime Minister. The current P.M., Petteri Orpo, was out of town and had told Marin that she could show me around. “This is very Finnish,” Marin said of her successor’s hospitality. “Even though we’re opponents, people are cozy.” She led me to the sauna—one of more than three million in the country—which was in a stand-alone cabin. “This was one of the few places I could relax during COVID,” Marin told me. “I’d come in here at 11 p.m. and just . . .” She trailed off, miming an exhale.

We wandered around the grounds, which overlooked the waters of Seurasaarenselkä. She pointed out a basketball court, where she used to shoot free throws to decompress. We walked into the main house, and several members of the house staff waved hello. Marin showed me into a dining area that doubled as a conference room and pointed up at the ceiling: “Whenever Emma used to play upstairs, the chandelier would shake.” She gestured out the window. I could see a small gazebo on the water’s edge, and she told me that she and Räikkönen had been married there, in 2020. The pair would split three years later.