A homeless woman complains that people walk right past her as if she were invisible, but one person who notices her is playwright Mark Scharf. His Baltimore Playwrights Festival entry “Scorpions” makes for a socially engaging evening at Fells Point Corner Theatre. Scharf’s play owes its unusual title to an old folk tale about a scorpion hitching a ride on the back of a frog as they cross a stream. Several characters in the play deliver monologues offering variations on this tale about whether the scorpion, which cannot swim, will sting the swimming frog, and how both creatures respond to a primal situation in which normally adversarial animals have struck a deal, of sorts. That cautionary story about a scorpion and a frog is not exactly subtle in terms of how the playwright periodically weaves it through an otherwise realistic contemporary drama set in a Washington, D.C. Metro station, an office, an apartment and a bar. This is a contemporary world that many spectators will recognize from their own lives. The playwright’s directness ensures that homelessness, the nature of friendship and other vital issues will get a full airing; but that blunt approach also leads to patches of stilted dialogue and a few borderline-implausible plot developments as the play delivers its stinging thematic points. To its credit, “Scorpions” does not simply rely on establishing a schematic divide between those with homes and those without. It’s obviously a crucial consideration here, but the play also wants to show how those blessed with homes and jobs often aren’t much nicer to each other than they are to the subway beggars they pass on their way to work. A homeless African-American woman named Mattie (Robin Rouse) is a blanket-swaddled and nearly mute presence at the entrance to a Metro station. Two downtown office workers who routinely pass by her are colleagues Derek (David Kellam) and William (Gary Sugai), who work for a company that’s unfortunately presented in such a generic fashion that it’s not quite possible to say what they do in front of those computer screens. The near-absence of workplace detail in the extended office scenes ultimately is not a big deal, however, because the heart of the play lies in the way in which Mattie enters their lives. Of equal importance is how this new presence in their lives affects the potentially unstable friendship between the African-American Derek and the Chinese-American William. When William befriends Mattie, it puzzles Derek and also Derek’s Lebanese-American girlfriend, Zaynah (Kalin Noel). After all, these are urban professionals whose social network does not extend as far as those without comfortable jobs and homes. Scharf adroitly develops the ominously tinged situation comedy potential of this awkward situation, which pays off in a humorously uncomfortable guess-who’s-coming-to-happy hour scene. Also to the credit of Scharf’s play is that Mattie is not merely presented as a silent object for our sympathy, because she quickly turns out to be as articulate and as culturally aware as any of the professionals who harbor stereotypical assumptions about her. Although it’s frustrating for them and, for that matter, the audience that Mattie’s biography remains rather hazy, it’s also viable in terms of how the play admirably allows for mysterious and even menacing elements in all their lives. The introduction of Mattie into the settled lives of Derek, William and Zaynah provides the play with plenty of sociological material. “Scorpions” always remains interesting and even provocative, but some of its plot twists and character turns seem unlikely. The often-forced dialogue also hurts. Just as the play does not always flow as naturally as it might, the capable performances sometimes feel a tad constricted and Sugai needs a firmer handle on his lines in a few scenes. It seems likely that the cast would be just fine if it were delivering more natural-sounding dialogue. Despite being broken up into numerous short scenes, the play moves smoothly thanks to director Miriam Bazensky’s tight focus on how two commuters make a connection with a homeless woman. It’s a situation that deserves the serious attention it receives here. Mattie is somebody worth noticing.