Boasting a breakthrough, award-winning performance by Brit Rosy McEwan (“The Alienist”), writer-director Georgia Oakley’s “Blue Jean” charts the dangerous and uncharted land traveled by a gay woman (a PE teacher at a public high school) in Newcastle in northeast England in 1988. At a time when Margaret Thatcher is calling for a return to “traditional values” and her supporters bemoaned the pernicious influence gay people could exert upon children, a gay teacher has to tread very carefully and even remain in the closet to safekeep her livelihood.

Thatcher even passed Section 28, a law prohibiting the “promotion of homosexuality.” It remained in effect until 2000. At school and in public, Jean Newman, who lives alone in a semi-attached property, is cautious and passive. A busybody across the way keeps an eye on Jean. In opening scenes, Jean, a bottle blonde, is applying bleach to her hair. With her fair skin and make-up free face, the effect suggests a desire to be invisible. At school, Jean, who was once married to a man and is divorced, is careful not to socialize with her female students and keeps her distance from her colleagues. She has been dodging going out for pint with a colleague on a Friday night for ages. In her favor, Jean is a talented pool player. One of the girls she teaches, Siobhan (Lydia Page), has a dangerous crush on her.

Jean prefers to socialize at the gay bar near where she lives, which is a distance from the school. She is involved in a serious relationship with Viv (Kerrie Hayes, “Black Mirror”), who is as liberated as Jean is imprisoned. Viv has a buzz cut, tattoos, piercings, leather jacket and… a motorcycle. She has been involved with someone like Jean before, and it did not end well. When Jean tries to watch the popular British show “Blind Date,” Viv tells her turn it off, calling it anti-gay propaganda.

“Not everything is political,” says Jean, who is about to learn that everything is. When a headstrong new student named Lois (Lucy Halliday) triggers Siobhan’s jealousy and then turns up at the gay bar in Jean’s neighborhood, things are about to get a lot more complicated for Jean.

“Blue Jean,” which is both a title and a description, is a deeply intelligent look at the difficulties even enlightened Western societies impose upon gay men and women. The film explores why a cold chill runs down Jean’s spine when she has the words “traditional values.” I have to admit that the elderly Tory (no doubt) we see on the news who uses the expression resembles the devil himself. It’s a bit of overkill. But the complications even ensnare Jean’s family. When Jean objects to a picture of her in her wedding dress on the mantle of her sister Sasha (Aoife Kennan), Sasha tells Jean that she knows she is gay. But Sasha also questions Jean about having a woman friend over when Jean is babysitting her loving 5-year-old nephew Sammy. Will Jean finally switch from “flight” to “fight?” “Blue Jean” is bravely over-endowed with loose ends. Oakley doesn’t pretend to have all the answers or even a few of them. The conflicts and issues raised in “Blue Jean” have not been resolved to this day. But wade in. McEwan is riveting as the beautiful, paralyzingly conflicted heroine. The score by Chris Rose (“Is It Safe to Be Gay in the UK?”) is even more nervous than she.

(“Blue Jean” contains nudity, sexually suggestive scenes, underage drinking and profanity)

Not Rated. At the Coolidge Corner

Grade: A-