"The Soap Myth," at the Black Box Theater Speeches by professional Holocaust deniers must have appeared in films and on television programs, but mercifully I had missed them. So Dee Pelletier's frightening monologue as Brenda Goodsen in "The Soap Myth," now at the Black Box Theater at the Harold and Miriam Steinberg Center, was a revelation. That's the sort of thing these people say? "The Soap Myth" is about the long-held belief that among the horrors perpetrated in the Nazi death camps was the manufacture of soap made from the body fat of Jewish corpses. This solid, thought-provoking production, from the National Jewish Theater, does not actually question whether that really happened, although characters do refer to accounts of it as myth and rumor. Instead, it asks if denying the highly probable truth of an old evil is justified in order to deflect a newer evil: people like Brenda Goodsen. As museum officials and historians tell our camp-survivor protagonist, Milton Saltzman (Greg Mullavey), since this particular crime against humanity was not well enough documented to be 100 percent provable without qualification, it just gives ammunition to the people who claim that the Holocaust never took place. A four-person cast tells the story, which amounts to Milton's campaign to convince the world of the so-called myth's reality. Ms. Pelletier is fairly uninteresting and stereotypically dowdy as a Holocaust museum official but seductive and disturbingly charming as the anti-Semitic speaker. Mr. Mullavey is gentle but angry (and almost unrecognizable from his television days as the affable husband in "Mary Hartman, Mary Hartman") as Milton. Donald Corren, who brought such versatility and worldly warmth to "Dietrich & Chevalier: The Musical" in 2010, is a chameleonic wonder as a historian, a Catskills comic and at least three witnesses at the Nuremberg war-crime trials. Andi Potamkin is lovely and dewy-eyed as Annie Blumberg, the young magazine journalist who writes Milton's story, but under Arnold Mittelman's direction, she seems to employ only two facial expressions (sympathetic and bewildered). On the other hand, she does convey a very believable, and unfortunate, youthful condescension toward her elderly subject. This is the second time around for "The Soap Myth," probably because a newer work by its playwright, Jeff Cohen, "The Man Who Ate Michael Rockefeller," was so well received last season. There has been considerable reworking since a 2009 production, but the play still feels a bit more like a discussion than like a drama. It is, happily, an astute one, with a genuinely moving conclusion.