If you wrote a novel featuring six sisters like the Mitfords — a notorious Nazi supporter, a noted communist, a blockbuster author of sparkling romantic fiction, and that's only half of them — you'd likely be told that your characters weren't entirely believable. Surely, an editor might harrumph, it isn't possible to have such extremes in one family. The very real Mitford sisters, who grew up in their family's posh but financially precarious pile in early 20th-century rural England with one clearly outnumbered brother, surely faced plenty of raised eyebrows in their lives. Though I doubt that oldest sister Nancy ever wryly intoned, as she does in the uneven but thoroughly watchable new BritBox series "Outrageous," anything like, "We can't have two sisters both mistresses of fascist leaders — what are the odds?," you can't blame the screenwriter for writing the line; it, like these sisters, is rather irresistible.

"Outrageous," a six-part series created and written by Sarah Williams (whose credits include "Flesh and Blood" and "Becoming Jane"), follows the Mitford sisters through much of the 1930s, as we see their personalities and ideologies shaping. (There's a 16-year age gap: Nancy was born in 1904; the youngest sister, Deborah, in 1920.) Nancy (Bessie Carter), who would later write "Love in a Cold Climate," hones her sharp authorial voice while struggling to find love. Pamela (Isobel Jesper Jones) finds pleasure and comfort in rural pursuits: tending chickens and enjoying country life. Diana (Joanna Vanderham), the family beauty, shocks all of England by leaving her wealthy young husband for notorious fascist leader Oswald Mosley (Joshua Sasse). Unity (Shannon Watson) moves to Germany and becomes obsessed with Adolf Hitler. Jessica (Zoe Brough) scowls through the photo session for her debutante ball and becomes a socialist in her teens. And Deborah (Orla Hill) — well, Deborah doesn't do much in this series, being quite young, which is just as well as there's more than enough plot already.

Based on Mary S. Lovell's 2001 doorstopper of a biography "The Sisters: The Saga of the Mitford Family" (though stopping about halfway through the book's events, leading me to wonder if another season is planned), "Outrageous" faces a challenge with its tone. Does it play up the Bright Young Things aspect of the sisters' young lives: the parties, the dancing, the music, the Champagne? Or focus more on the darkness, and the horror of watching two young women disappear into the repellent ideology of the Third Reich? The series opts to do both, to sometimes disconcerting effect; there's a playfulness to much of the narrative (arch voice-overs, perky music, silly title cards saying things like "Meanwhile, somewhere in Gloucestershire") that marries awkwardly with scenes depicting Nuremberg rallies and "Juden Verboten" signs outside German restaurants. It's not inaccurate; the Mitford sisters, in the 1930s, wouldn't have entirely known what horror lay ahead. But we do, and that knowledge makes some of the scenes hit perhaps harder than intended — or harder than this mostly jaunty series can sustain.

Nonetheless, "Outrageous" is catnip for those who love period films and strong female characters. The standout is Carter's Nancy, whose delightfully astringent voice narrates much of the episodes; a fascinating character who's at once smart, funny, self-confident and utterly hopeless at love. (Carter, previously known for "Bridgerton," is the talented daughter of actors Jim Carter — Carson on "Downton Abbey" — and Imelda Staunton, and you can see and hear both of them in her performance.) Also vivid is Vanderham, who gives the uncannily polished Diana a meticulously slinky walk — this is a woman who knows she's being looked at — and a cool determination, as if she's willing herself to not see what's in front of her. Watson's Unity has a glassy, not-quite-there quality to her; we're never sure (and perhaps that's intentional) how much of Unity's conversion to the Nazi cause is genuine and how much is due to unspecified mental illness. And Anna Chancellor and James Purefoy are a delight as the sisters' often perplexed parents, known as Muv and Farve. "Each one of those girls is more perverse than the other," notes Farve at one point; he's not wrong.

"Outrageous" works best when it digs deep into the relationships between the sisters, into the question of whether you can love someone who's disappeared into darkness — but is, nonetheless, still that person you grew up with. "Perhaps you don't get a choice about loving sisters," muses sensible Pamela, midseries. "Maybe the love's just there in the background and always will be, whether we like it or not."

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'OUTRAGEOUS'

Rating: TV-MA

How to watch: BritBox

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