Osborne’s descriptions of life in upper-class Edwardian London, “an age overflowing with millionaires,” are bracing. Adultery is commonplace. The country enters World War I, and while husbands and entire families of sons are obliterated on French battlefields, Sackville’s set is desperate for fun, injecting morphine, attending “bottle parties” and downing vats of White Ladies, whiskey sours and Bronxes. (“The Bolter” does double duty as a history of cocktails.) Osborne vividly describes a social class unhinged by too much money and too much death. By 1921, “nudity was all the rage.” Women were dancing on tables, wearing transparent dresses. One hostess greeted her guests “wearing nothing but a famous string of family pearls which reached her pubic hair.”

Image Idina Sackville and her third husband, Josslyn Hay, 1923. Credit... Courtesy of the 24th Earl of Erroll; from “The Bolter”

Sackville’s husband falls in love with the architect Edwin Lutyens’s daughter, prompting Sackville’s first bolt. From there the narrative moves to Mombasa, where Osborne deftly sketches in the milieu of a colonial empire, with its 600-mile Iron Snake, the Uganda Railway, which would make it possible for the British to control Egypt and the Nile. “All that was needed was farmers,” Osborne writes, “for no fewer than two and a half million acres needed tending to. . . . Given the unwillingness of the indigenous people to surrender their lands, all the better, went the thinking, that the territory should be occupied by men who knew how to handle a rifle.” With the end of the war, Britain was flooded with soldiers who wanted a new life; Africa was “an earthly paradise” whose landscape was “genuinely familiar, indeed almost Scottish,” and would provide food for a hungry Europe.

Africa became Sackville’s true love. Over the course of five marriages, she would build — and leave — three farms, working side by side with the African laborers. “Like the local Kikuyu tribesmen and much to their amazement,” Osborne writes, “she both walked and rode barefoot” over thorny fields. But she also kept servants, rode out on safari for weeks at a time and partied hard. She thrived on sexual adventure and set up a mirror over her bed so as not to miss anything. Osborne is fascinating on the social rounds of this new African empire. We catch glimpses of the Sitwells; Cecil Beaton; Stephen Spender; and Beryl Markham, Karen Blixen and their lover Denys Finch Hatton. People met at weekend house parties, races and livestock auctions. Sackville was a magnet. Within several years of her arrival, there were so many partners changing beds that it became a political scandal, with the British government appalled at the inability of the colonial administration to control the miscreants. “The joke ‘Are you married, or do you live in Kenya?’ was doing the rounds.” The Crash of 1929 seems to have done little to stop the dizzying frivolity. The spending never stops. And the houses! The extravagance of the furnishings!

At some point, as the Happy Valley crowd sank into an addled haze, I wanted to cover my eyes — probably during the “sheet game”: men would hide behind a sheet strung across the room, circles were cut into it through which a woman would grope a hand, a nose, an elbow, to identify the owner; as the alcohol content went up, the holes in the sheet were cut lower, and the men unbuttoned their trousers.

Sackville is finally reunited with her eldest son, but the chaos does not end. World War II arrives and Sackville’s newfound connection to her family is cut short. The last decades of her life are unbearably sad; she seems cursed. It can be hard to appreciate Sackville’s incessant charm, her ability to engage and seduce anyone she wanted, her generous and kind nature, and her capacity for lifelong friendship. But the reader falls under her sway, too.