In HBO’s “Catherine the Great,” Mirren portrays the 18th-century Russian empress at the height of her powers.

In the first episode of the HBO mini-series “Catherine the Great,” the empress of Russia is hunched over her desk writing about her political ambitions while her jealous lover stares into a hand mirror, primping for a cross-dressing masquerade at the palace. “It just feels like something has changed between us,” he says, wearing a golden scoop-neck gown, his makeup half-done. Catherine (Helen Mirren) wears a man’s suit with a tricorn hat that makes her look like an American revolutionary. She rises from her desk, strides over to her male companion and, in a few succinct sentences, puts an end to their yearslong romance. “You want something I cannot give you,” Mirren says simply. “You want power.” Her lover, Count Orlov, who helped engineer the coup that unseated her husband, Peter III, thinks it is time to marry her and take on a more powerful role in her regime. But Catherine sees an army of men encircling her, plotting ways to co-opt her authority, and she has no intention of giving it up. “They totally underestimated her,” Mirren said in an interview, “From that moment on, she knew she had to control the male aristocrats around her. And she did. She did by outsmarting them.”

The life of Catherine the Great has long been fodder for cinematic period dramas. But in the past, filmmakers chose to center the story on Catherine’s early life — how she came to rule an empire rather than the three and a half decades she spent ruling it. In one of the most prominent films about her life, “The Scarlet Empress” (1934), Marlene Dietrich played a young, wide-eyed German princess who is summoned to Russia to marry the future monarch. In 1991, Julia Ormond played a similar character in the mini-series “Young Catherine,” and four years later, so did Catherine Zeta-Jones. But the four-part mini-series, which debuts on Monday, focuses on the ruler at the height of her power. The series tracks Catherine over the course of her 34-year reign, as she evolves from an idealistic, vulnerable usurper warding off enemies on all sides into a tyrannical autocrat who no longer dreams of liberalizing the country. At the heart of the story is the empress’s turbulent romance with Gregory Potemkin, the man historians view as the greatest love of her life. “This part of the story was more interesting than the young girl at court trying to find her way,” said Nigel Williams, who wrote the series. “This is a story about a woman who is incredibly powerful and exercises that power with considerable tact and finesse.”

Williams said the choice to focus on Catherine in her later years was partly driven by the desire to cast Mirren. At 74, she has had plenty of practice playing a monarch: She won an Academy Award in 2007 for her portrayal of Queen Elizabeth II in “The Queen,” and before that, she starred in the British mini-series “Elizabeth I,” which collected several Emmys. Mirren also has deep familial ties to Russia — in a sense she is more Russian than Catherine herself, who was born into an aristocratic German family as Sophia of Anhalt-Zerbst. Mirren’s father was born in Russia and her grandfather was a colonel in the Russian army during the Russo-Japanese War. During World War I, he was asked to travel to Britain on a diplomatic mission, and the Russian Revolution of 1917 prevented his return home. “The reason I exist in Britain was because he was sent by the czar to make an arms deal with the British government,” Mirren said, “and basically got cut off by the revolution.”

Catherine, who ruled from 1762 to 1796, presided over the empire’s territorial expansion and growing recognition as a global power. The HBO series touches on some of the notable events of her reign, including the country’s wars with the Ottoman Empire, a peasant rebellion that swept through the countryside, and Russia’s annexation of Crimea, which continues to shape international politics even today. But the only thing most people seem to know about Catherine is her “sexual profligacy,” Mirren said, a reputation the actor considers to be unfair and misogynistic. (Catherine’s contemporaries enjoyed not only gossiping about the empress’s sex life but fabricating crude stories about it, including a persistent false legend about a final, fatal sexual encounter with a horse.) Catherine had a dozen lovers throughout her reign, many of whom were much younger and were given an official designation of “favorite,” collecting lavish gifts throughout their tenure, according to Robert K. Massie’s 2011 biography “Catherine the Great: Portrait of a Woman,” which the screenwriter Williams read as part of his research. But Catherine was hardly exceptional for this. The system of taking “favorites” was already accepted practice in the Russian monarchy when a 14-year-old Catherine arrived in Russia and the Empress Elizabeth was in power, Massie wrote.

“She just behaved like a man would, except she was a romantic,” Mirren said. “She loved to be in love. She was a serial monogamist.” Given the empress’s two-dimensional reputation, Mirren wanted to introduce a nuanced portrayal of Catherine to a mainstream audience, she said. The series shows Catherine from multiple angles, with her many contradictions on display: the devoted romantic juxtaposed with the active admirer of young men, the liberal idealist with the controlling authoritarian, and the diligent workaholic with the woman who declares that all she cares to talk about is “sex or gardening.” Which isn’t to say the show ignores the empress’s well-documented carnal desire. In typical HBO fashion, the series brims with sex. This monarch is a far cry from the buttoned-up Queen Elizabeth II, whom Mirren portrayed as a restrained traditionalist enraged by Princess Diana’s displays of independence. Mirren found herself taken aback by Catherine’s flouting of sexual mores. In one scene, the empress bluntly asks her much-younger personal secretary to “do whatever it is you think I want”; in another, she’s winding down from an explosive political quarrel in her palace when she declares that what she needs is “a good screw,” using more colorful language.

“I came to my youth in the post ’50s era where there was still that awful, repressive, restrictive attitude,” Mirren said. “When I first started looking at the history, I thought, I can’t get my head around this, actually. Because I’m still very much a child of my upbringing.” Despite Catherine’s parade of partners, the series is centered squarely on the romance between the empress and Potemkin, played by the Australian actor Jason Clarke. In the series, you see their playful courtship, their declarations of love, their private yearnings and their frequent and fiery public arguments. The couple’s relationship rarely has been explored onscreen in such depth; filmmakers more typically focus on Catherine’s marriage, at 16, to her abusive, maniacal husband, Peter III. (Peter III died in custody not long after being deposed, officially from illness, but some theories implicate Catherine’s allies in his death.) Clarke said he saw Potemkin as a man who, like others before him, sought to gain power through his relationship with the empress but also truly loved her. In preparing for the role, Clarke relied mostly on surviving letters from the lovers. “The connection to each other was very deep,” he said in an interview. “They used the playfulness of their words to spur each other on. He cared for her.”

In those letters, Catherine called Potemkin pet names like “dear husband” and “twin soul,” showering him with love and begging him to return safely when he was off seeking to expand her empire, Massie wrote in the biography. She also called him names like “general” and “prince” as she continued to give him more power in her army and her council, behavior that increasingly angered the aspirants around her with each promotion. The relationship between Mirren and Clarke feels subversive onscreen because of their inverted power dynamic — she as the all-powerful empress and he as the devoted political servant — but also because of the actors’ ages. Mirren is almost 25 years older than Clarke, who is 50. In reality, Catherine was 44 when they began their affair, and there was only a 10-year age gap. (That’s just one example of how the series knowingly bent the historical truth for the story’s sake, Williams said.)

The romance between Catherine and Potemkin was “operatic,” said Philip Martin, the director of the series, and he found scenery to match. The cast and crew spent about two weeks filming in St. Petersburg, the former Russian capital, where they were allowed limited use of historical sites like the Peterhof Palace. In one scene, filmed in Catherine Palace outside of St. Petersburg, after receiving a letter with bad news, Catherine collapses, her dress billowing on the wood floor beneath her. That floor, Martin said, is both the original floor that Catherine walked on during her reign and the one on which Nazi soldiers built a fire during World War II, as evidenced by scorch marks. As the series progresses through the decades and Catherine enters her late 60s, she is given to musing about the meaning of her life. While she begins her reign energized by her ambition to free the millions of Russian serfs, by the end she has long given up such liberal ideals in favor of expanding her empire. (Shades of another recent HBO monarch: Daenerys Targaryen on “Game of Thrones.”) She has tyrannical outbursts, and she remains hypersensitive to the dangers around her. Martin, who directed episodes of “The Crown” for Netflix, said that he wanted to explore onscreen how Catherine became the iconic figure she was and how, at the end of her life, she grappled with the choices she had made.

“It’s about the consequences and costs of holding onto power,” Martin said. “It’s a journey of seeing this woman change over time and being with her at the end and trying to figure out what the journey meant.”