A mushroom cloud looms over London, millions of citizens incinerated, the radioactive ash descending on the rubble.

In October 1962, at the height of the Cuban Missile Crisis, 13-year-old Sally Potter saw this doomsday scenario -- every night in her dreams.

"It's very interesting, those people who can directly remember the crisis," says the filmmaker, whose beautiful "Ginger & Rosa" -- about two London teenagers, fast friends caught in a whirl of personal and political tumult -- is set during that fateful fall, when the whole world looked as if it were going to go ka-boom.

"When you did live through it and you did have nightmares, it was something real. It's not just a historical event. It felt very close."

The world, of course, did not go ka-boom. But Potter never forgot her dreams, or her dread.

In "Ginger & Rosa" [at Images Cinema in Williamstown], Elle Fanning plays a 16-year-old Londoner obsessed with nuclear apocalypse. As Ginger, Fanning listens to the nightly newscasts, the brinkmanship between Moscow and Washington, the estimates of fatalities. Ginger joins ban-the-bomb marches. And she drifts apart from her best friend, Rosa (Alice Englert), at just the wrong moment, when she needs her most.

"I wanted to work with an intimate feeling of catastrophe," Potter explains. "Of a personal world ending, or breaking apart, and have that echoed by the wider world potentially ending and falling apart."

Living and working and going to school in Southern California, the younger sister of actress Dakota Fanning not only convincingly plays a Brit, but also convincingly plays a teenager faced with a perfect storm of crises.

"I thought I was probably going to cast an unknown girl from the U.K.," the filmmaker says. "I saw 2,000 girls via the Web, and a couple of hundred girls did more conventional auditions. And then my casting director in Los Angeles sent me a little tape of Elle, and I thought, ‘Uh-oh. ‘

"I flew over to L. A. and worked with her. I got the feeling for how she could work, what she could deliver, what the whole thing might be like, and it was a complete goose-bump situation. I knew immediately she was the right one."

Fanning was 12 when she and Potter met, 13 when they started shooting.

Fanning isn't the only outside-the-box casting choice in "Ginger & Rosa." Englert is Australian -- the daughter of director Jane Campion. And Christina Hendricks -- yes, Joan of "Mad Men" -- stars as Ginger's unhappy artist-turned-housewife mom.

"Of course, I absolutely love her in ‘Mad Men,' and I was really fascinated that this whole different side of her emerged," Potter says. "And then you realize that actors get so trapped in one part of their work, even when it's great. So it's always very nice to work with somebody when you can offer them something different."

"Ginger & Rosa" is something different for Potter -- a deliberately straightforward narrative.

"I thought, how about doing something radical? Be straightforward," she says, laughing. "It's a challenge because I very easily get seduced by the vitality of pushing the medium forward."

And so, in "Ginger & Rosa," "I gave myself the instruction to restrain myself," Potter says. "Be more simple. And of course, simplicity is the hardest thing, actually."