Jeff Still (Howard), Shayna Blass (Charlotte), Xavier Scott Evans (Jonny) and Emily Townley (Lucinda) in “The Mystery of Love & Sex.” (Margot Schulman)

Black/white, Jewish/Christian, gay/straight — which are the markers that matter? The identity minefield is intensely charted territory, but playwright Bathsheba Doran navigates it with unusual grace in the entirely enjoyable play “The Mystery of Love & Sex.”

Doran’s writing credits include TV’s “Masters of Sex” and “Boardwalk Empire,” yet “Mystery” actually feels a little more like a novel as it sweeps across five years, watching a 21-year-old ponder what on earth it might mean to be a whole person. The scenes are detailed and unhurried, and in the smartly cast Signature Theatre production, you come to know each of the saga’s four characters well.

Charlotte, the white 21-year-old, seems to be attracted to Jonny, a black friend she’s had since they were 9. So, her parents wonder during a comical dinner in her cramped dorm room: Is this serious? After the adults leave, Charlotte soberly asks more or less the same thing, while adding a wrinkle. She confides to Jonny that, um, maybe she’s gay.

Jonny’s a Baptist and a virgin, despite the naked suggestion by Charlotte, who is Jewish (the brief nudity here is touching and amusing). The domestic confusion escalates as we learn about Charlotte’s parents: Howard is a Jewish author of detective fiction, and Lucinda is a Southern belle and lapsed Catholic who’s trying to quit smoking. The plot thickens even more as Doran contemplates Jonny’s longtime role around this family.

The acting in Stella Powell-Jones’s production is earthy and entertaining – it’s a funny show, yet it always feels honest enough that you can’t be sure how tragically things might turn out. Shayna Blass, an actress who is proving to be among the city’s most reliable performers, is wonderfully nuanced as Charlotte, whose impulses are all over the place. Blass makes this feel natural, even when the prickly Charlotte tries to unwind and dance, one of Blass’s many endearing moments. It’s easy to believe what a mess this kid is, and Blass thoughtfully never pushes our affections away from Charlotte.

Shayna Blass (Charlotte), Jeff Still (Howard) and Emily Townley (Lucinda) in “The Mystery of Love & Sex” at Signature Theatre. (Margot Schulman)

As Jonny, Xavier Scott Evans is tense in an entirely different way. Evans is excellent with Jonny’s tight, earnest talk, whether arguing with a demanding Charlotte or nervously squaring off with Howard. The parents gradually emerge in full color after starting out in sitcom style. Jeff Still crackles with spontaneity as Howard, bubbling with anxious support for Charlotte and counterpunching in self-defense about his writing. As Lucinda, Emily Townley is perfection: Doran arms Lucinda with most of the one-liners, and Townley’s comic timing is as good as it gets. Lucinda also articulates Doran’s cosmic views on life’s big mysteries, and Townley — especially in a sweet but difficult scene with Blass — lifts the show into its most expansive realm.

Doran, who was born in England and lives in Brooklyn, has received mixed reviews for her script in Los Angeles (“soap opera”) and at New York’s Lincoln Center for the Performing Arts (“tender and funny”), where Tony Shalhoub and Diane Lane played the parents and apparently he was the funny one. This production is comfortable, with understated costumes from Ásta Bennie Hostetter and a spare design from James Kronzer — whose movie house set for “The Flick” next door in Signature’s Ark theater, by the way, is wildly good — that efficiently creates a lonely, upscale world. The actors, in particular, may have captured the most convincing tone of “Mystery.”

The Mystery of Love & Sex by Bathsheba Doran. Directed by Stella Powell-Jones. With David A. Schmidt. Lights, Jesse Belsky; sound design and music remix, James Bigbee Garver. About 2 hours and 15 minutes. Through May 8 at Signature Theatre, 4200 Campbell Ave., Arlington. Tickets $40-$89, subject to change. Call 703-820-9771 or visit