What in the world is the matter with Jane Fonda?

That question, posed by President Richard M. Nixon in 1971, opens a new HBO documentary about the actress and activist. It’s safe to say that the answer, nearly 50 years later, is: Nothing.

Resplendent in a subtle animal print, a mop of blonde curls and still-electrifying blue eyes, Fonda submits to one of myriad interviews about the film with the aplomb, humor and magnetic physical beauty that have made her a sex symbol — again — at 80. The woman who has seemed to be inside our heads her entire lifetime is relaxed and direct, asking a reporter about her toenail polish before getting down to business.

“Is that Malaga Wine?” she inquires in her distinctively silky-husky voice. “Because that’s what I use, too.”

The immediate temptation is to go all girly on Fonda, to get all the tea about diets, exercise regimens, the name of her plastic surgeon. If “Jane Fonda in Five Acts” is any indication, she wouldn’t hesitate to share. Directed with sensitivity and ingenuity by Susan Lacy, the documentary, which begins airing Monday, transcends the usual celebrity biopic, eschewing a then-that-happened chronology and ­Wiki-esque factoids to deliver a disarmingly candid, emotionally profound portrait of a woman whose private and public lives have uncannily chimed with America’s. If there are indeed several acts in any well-lived life, Fonda’s fifth (surely not final) has been a bracing example of newfound autonomy and self-possession. Single after being married three times, steadily working after vowing she was done, she’s finally “moved back into my own skin,” she says, becoming “who I was supposed to be