A line from “All’s Well That Ends Well” is repeated throughout Anna Ziegler’s play “BFF”: “Our remedies oft in ourselves do lie.” Does that mean the protagonist needs to take responsibility for her past and her future, or that she just needs to get over her guilt?

The plot of the unfortunately titled “BFF” — that’s shorthand for “best friends forever” in the lingo of high school yearbook signatures — hangs on a compelling psychological mystery that deepens as the play progresses, and ends with a genuinely gasp-inducing moment that raises the stakes of everything that came before.

Ms. Ziegler’s script switches back and forth between scenes of the early teenage friendship between Lauren and Eliza, and a romantic relationship between a 20-something Lauren and Seth, a redhead she meets in a yoga class who is trying to find his “inner voice.” The relationships arc in rough tandem, but the parallels are never too neat.

Lauren and Eliza are first seen by the side of a backyard pool playing with each other’s hair and talking precociously about the future. The dialogue can be cloying, especially in these opening scenes, in which they moon about saying such things as “I sort of wish we could stay here forever,” which no preteen has said in all of human history.

While Lauren starts to wear makeup and go shopping and figures out how to navigate the school’s social landscape, Eliza stubbornly wears ripped overalls and lets her red hair hang loose and unkempt.

As Lauren begins to date a boy named Jason, Eliza tells her she shouldn’t indulge her vaguely sexual daydreams about him. Jason isn’t seen on stage, but Lauren relates stories of their “tongue kissing” at a school dance to a repulsed Eliza, who says they are far too young for such things, and, by the way, that Lauren dresses like a “slut.”

Lauren, meanwhile, calls another girl at school a “whore” for having perfect skin, and sleeps with Jason in his brother’s bedroom “with no candles” — ah, high school, don’t you wish you could have stayed there forever?

Meanwhile, the grownup Lauren is an emotionally fragile marine biologist. When she first meets Seth and he asks her name, she blurts out “Eliza.” In sitcom fashion, she doesn’t simply correct herself, but begins dating him under an assumed name. Like the teenage Eliza, Lauren has the frustrating quality of frequently bursting into tears and incoherent, rambling rants. The difference between Seth and the teenage Lauren is that Seth has the stomach for it.

Seth, played by the charming Jeremy Webb, is a 21st-century saint: self-aware, self-deprecating, and absurdly patient with Lauren’s neuroticism. A modern man, when faced with a woman who answers a simple question with an ominous “I don’t like invasions,” would reasonably flee. But Seth is unruffled, even as Lauren becomes more cryptic as their relationship progresses.

When he is a half hour late in meeting her for a casual date in a park, she explodes in anger but can’t articulate what’s wrong. When she ignores him for a full week, he talks to his therapist and leaves long, nonthreatening voicemail messages for Lauren. “I won’t beg you to call me back,” he says, and almost gets up the strength to hang up there and retain his dignity. He can’t hold it, though, adding: “But why not just call me back?”

She doesn’t, at least not until some damage has been done. By then, back in the teenage storyline, Eliza and Lauren have had a breakup of their own, as teenage girls often do. Eliza suffers mightily for not expressing sufficient interest in tongue kissing and lipgloss.

The play’s therapeutic language might suggest that Lauren should simply “forgive herself” for the fallout of both friendships, but Ms. Ziegler’s script and Josh Hecht’s direction are thankfully more complex than that. Instead, we are left with the notion that maybe sometimes we feel guilt because we are guilty.

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