Lynn Redgrave Photo by Joan Marcus.

Because, within Redgrave's largely imagined yet emotionally true saga lies the understanding - or the lack of it - of the family that defines who we all are. As with her previous one-woman show, Shakespeare for My Father (which dealt with dad Michael), and her multicharacter play The Mandrake Root (loosely based on her mother, and Beatrice's daughter, Rachel), Redgrave delivers a poignant but probing look at a catalytic force in her life. This one, however, is perhaps made even juicier by the fact that Redgrave barely knew Beatrice herself and came to most of her discoveries and conclusions later.

Spurred by divorce from her longtime husband John Clark, cancer, her mother's death, and her brother's near-fatal heart attack, Redgrave flew to her grandmother's grave to search for inspiration, only to find that her name and the dates of her life had been eroded by acid rain. With them disintegrated any concrete understanding of the woman without whom the celebrated Redgrave acting clan we know today would be nonexistent. In an attempt to make sense of the unknown, Redgrave therefore does what any loving granddaughter would do: she makes up Beatrice's life.

Well, not entirely. Shreds here and there can be tied to reality, and certain facts along the way - mostly having to do with names and generalized outlooks on the pre-World War I world - are undoubtedly indisputable. But Beatrice's speech, the depth of feeling, and her specific experiences as a woman growing up at the threshold of what we consider modernity are new, backfilled recollections that make sense of the life and world Redgrave herself understands, but that in the strictest sense of speaking aren't traditionally accurate.

Beatrice's religious upbringing in Victorian England. Her isolation from any talk or simple knowledge of sex until her wedding night. (And, well, technically not even then.) The scandalous prospect of living without a maid, despite being completely unable to afford one. The entrapment within a marriage that she doesn't find at all emotionally satisfying, but that's ornamented with the birth of a son, Robin, who bestows on her a love and tenderness she'd never thought possible but which - thanks to the ravages of history - is not destined to last. Her reaction to her daughter and Michael's flourishing acting careers.

Because of the great care with which Redgrave approaches all of these anecdotes, it rapidly ceases to matter which are based in fact and which supposition - they're all moving and convincing nonetheless. With the merest of vocal changes - Beatrice's voice sits perhaps half an octave below Redgrave's - and few physical movements to speak of (for all intents and purposes, Redgrave never rises from behind that desk), the actress carves a woman of astonishing personal fortitude and character who was, in her own way, just as theatrical as the better-known figures from her famous family.

Director Joseph Hardy helms the minimalist but effective production, keeping the set design (by Tobin Ost), limited to that antique desk and a postcard backdrop, and the lighting (Rui Rita) as simple as can be, but Redgrave's emotions and delivery complex. Despite the lack of trappings, you never feel you're seeing any sort of a lesser venture - only one woman at the top of her form, embodying a gloriously fictional real-life woman - or is that a gloriously realistic fictional woman? That it's hard to tell for sure is one of the play's most telling joys.

Though you never have trouble distinguishing Redgrave from Beatrice, after a while the two begin to meld into one - that's as it should be. The play's clearest through line is that despite having hardly known each other and being born some five decades apart, the two are kindred spirits of a kind. The former was strong-willed but constricted by the delicacy of her time; the latter has had more, but not necessarily better, options for conducting her life, and has faced much of the adversity that Beatrice spent decades silently fighting against.

By the end of the play, when Redgrave most openly acknowledges the value of all she learned from her grandmother - whether real or not - you're deeply reminded of the impact that your family (even those you've never met) have had on your life. Redgrave is apparently a new character in her own story since the play's earlier productions at the Mark Taper Forum in Los Angeles in 2006 and Hartford Stage in Connecticut in 2007. But because she's so instrumental in conveying who Beatrice was and what she stood for, it's now impossible to imagine Nightingale ever properly unfolding without Redgrave standing right by her side.

Nightingale
Through December 13
New York City Center, 131 West 55th Street, between 6th and 7th Avenues
Tickets online and current Performance Schedule: New York City Center