If you think social media is a cesspool, Mary Lee knows that it’s even worse than that.

Her story, recounted in Anna Moench’s play “Sin Eaters,” starts like the internet did: with the promise of a brighter, improved future. Mary (Bi Jean Ngo) and her partner, Derek (David M. Raine), are celebrating; she has finally landed a new job, and in tech at that. So what if she found the gig on Craigslist, it’s temporary, it pays $20 an hour, and she doesn’t know exactly what her duties will entail? It’s money, which the couple need to move out of their Staten Island hovel.

“Sin Eaters,” presented on-demand by Theater Exile in Philadelphia, kicks off as a standard domestic dramedy. Derek, who has artistic aspirations, sulks a bit when Mary Lee points out that it would be easier for them to find a new place if they had two incomes, and suggests he should go back to catering.

Darker waters, however, are churning underneath the banter. Noises from the neighbors bleed into the couple’s apartment, alternately gross and ominous. The petulant Derek has an unwelcome passive-aggressive streak. At one point, he adjusts a home surveillance camera on the ceiling, and it’s unclear whether Mary knows it’s there.

The unease grows more sharply defined when Mary turns up at her new cubicle (Matt Pfeiffer’s deft, inventive staging for this virtual production makes the most of the two main sets). She has been hired by a new social media platform, Between Us, to review anonymous posts that have been flagged for guideline violations. As anybody who has ever taken a wrong turn on the internet can attest, it does not always bring out the best in people.

Mary’s days are a parade of gore, racism, child abuse and animal torture — a list of no-no’s helpfully hangs on a whiteboard, a constant reminder of the horrors people are capable of. “It’s a hard job,” her supervisor, Steve (voiced by Raine), tells her. “You eat the weirdos’ sins so normal people don’t have to.”

Moench, whose play “Mothers” also displayed a penchant for dark humor, has set up a great premise. And the first half of “Sin Eaters” moves with assurance, layering paranoid, unsettling vibes and satirical barbs targeting contemporary corporate environments; the winner of a productivity challenge gets to choose between a $50 Starbucks gift card and a Skype interview with the company’s content managers.

The play is on less solid ground when Mary’s job inevitably gets in her head. In theory, what happens on Between Us stays on Between Us; but the web has a nasty habit of spilling into real life, and vice versa. Just when Mary is getting used to — or rather, desensitized by — her daily parade of depravities, she thinks she recognizes somebody in one of the videos under review. She is not 100 percent sure, though, especially since everybody around her starts looking familiar. (Raine, who is Ngo’s real-life partner, plays all the supporting characters.)

The turn into psychological horror — shades of Tracy Letts’s “Bug” or the Roman Polanski film “Repulsion” — feels a little tentative, in both the writing and direction. Still, it’s refreshing to see a play embrace genre instead of snobbing it as if it were the equivalent of a catering job.

Sin Eaters
Through Feb. 28; theatreexile.org.