Who is the powerful, sexually wise yet also mysterious phenom that is Dr. Ruth? Director Ryan White opens his fascinating Ruth Westheimer portrait by posing that question in a contemporary way, having the doctor herself ask Alexa, in the kitchen of her NYC apartment where she has lived for 54 years. Ask Dr. Ruth nicely fleshes out the back story and undercurrents of the TV personality and America’s frank-speaking “sex-pert” by cross-stitching chronology in a tale spanning her German-Jewish parents’ Holocaust death, nearly losing her legs while a sniper for the Jewish paramilitary Haganah, raising a family, and becoming a pioneering sex therapist. Her media notoriety began in 1981, with the radio advice show “Speaking of Sex” on WYNY, which launched her meteoric rise to fame that included a whirlwind of talk show appearances, her own TV series, and even a board game. The film ostensibly ends with her 90th birthday party celebration, but, like its subject, refuses to quit: Stay for the spunkily charming Jewish mother/sage/quipster’s outtakes over the end credits.