Joe Estlack and Ayelet Firstenberg Photo by Jessica Palopoli

These—and many other—questions are raised in Lucy Prebble's fascinating and complex The Effect, now playing at San Francisco Playhouse, directed to thrilling effect by Bill English.

The Effect tells the story of two volunteers in a month-long live-in pharmaceutical study of the efficacy of a new anti-depression drug. Tristan (Joe Estlack) is seeking some seemingly easy cash to fund his wanderlust, and Connie (Ayelet Firstenberg), a psychology grad student, seems to have signed up out of intellectual curiosity. They meet and immediately discover two things in common: they share a birthday and bite their nails, both of which Tristan takes as signs there is something bigger in store for them. Beyond that, they don't seem to have much chemistry. Until, that is, the pharmaceutical company delivers its chemistry in the form of pills they down (in increasingly higher doses) with clockwork precision under the watchful eye of Dr. Lorna James (Susi Damilano), the psychiatrist leading the study. (She herself is under the watchful eye of Dr. Toby Sealey, played by Robert Parsons, but more about him later.)

Dr. James conducts the initial assessment interviews with clinical precision and emotional detachment. (Damilano is wonderful in the role, maintaining a chilly professional surface demeanor, while allowing the passionate, committed healer inside to peek out from time to time. It's unlike anything I've seen Damilano do, and it's marvelous to see her range.) She grills the two (separately) on everything they're not—diabetic, asthmatic, pregnant, etc.—ignoring who they are. That we must discover when they are alone together, and Connie discovers some of the same ardor for Tristan that he has for her. But is this new sensation truly hers, or is it sparked by the drug they are taking? (Or not, since one or both of them could be receiving a placebo.)

Playwright Prebble's storytelling skills are on full display here. There's nothing experimental or deliberately obtuse about the plot of The Effect. It has a beginning, middle, and end. In that order. With twists that happen at just the right places to suck you deeper into the narrative. Her characters are well drawn, with nicely placed details to help us to both understand their motivations and fill them out as fully dimensional humans.

In addition to Damilano's image-breaking performance, Firstenberg and Estlack do equally powerful, revealing work in their roles. Firstenberg's Connie is relentlessly curious, rational and vulnerable, once the dopamine—whether her own or supplemental—kicks in. As free spirit Tristan, Estlack exhibits a wonderful youthful energy and enthusiasm, at least until late in act two, when Prebble turns The Effect in a much darker direction. Estlack and Firstenberg have a wonderful tense chemistry, and director English has staged a fascinating series of brief scenes of their post-coital pillow talk that exploits it to wonderful effect.

I only wish Robert Parsons' (as chief doctor of this fictional pharmaceutical company) and Susi Damilano's characters had a relationship that felt half as natural. I'm not sure if Parson is miscast or simply not up to the task, but his portrayal of Dr. Toby Sealey lacks the sense of gravitas and menace his character requires. Dr. Sealey is a laser-focused manipulator, but Parsons' portrayal makes him come across as a rather whiny milquetoast, and not the powerful tool of big pharma I believe Prebble intended.

Technically, the production is stellar. Once again, Nina Ball has created a wonderful environment in which the action takes place: an almost sterile atmosphere primarily of white and chrome, like something out of a Star Trek movie. Lighting by Kurt Landisman is appropriately illuminative (sometimes glaringly so, but that's right in tune with the environment), and Theodore J. H. Hulsker's sound design creates a level of richness that adds dimension to every action on stage.

At one point, Estlack's Tristan cogently sums up the issue at the heart of The Effect. When Connie questions the sincerity of men who have a hard time saying "I love you" without a few drinks in them, he responds that "Men mean that when they say it. They just can't say it unless they're drunk." Is love something chemical, or do chemicals (whether ethyl alcohol, Prozac, or naturally occurring dopamine) merely serve as the trigger for something more deeply—and more uniquely—human?

The Effect, through April 28, 2018, at San Francisco Playhouse, 450 Post Street, San Francisco CA. Performances are Tuesdays-Thursdays at 7:00pm, Fridays-Saturdays at 8:00pm, with matinees Saturdays at 3:00pm and Sundays at 2:00pm. Tickets are $20-$125, available at www.sfplayhouse.org or by calling the box office at 415-677-9596.