We’re accustomed to origin stories in superhero movie franchises, but here’s one unlike any other. Last summer’s “Wonder Woman” might have the Amazon warrior’s de facto premiere in the so-called DC Universe (although the earlier “Dawn of Justice” gave us a glimpse of her in action), but “Professor Marston and the Wonder Women” tells the intriguing story of the character’s creation back in 1941. Angela Robinson’s film turns an edgy, quite radical tale—which one can investigate in detail via Jill Lepore’s 2014 book “The Secret History of Wonder Woman,” or in the articles she wrote for The New Yorker and The Smithsonian—into a rather blandly conventional crowd-pleaser, but in its glossy, unthreatening way it’s still reasonably engaging.

Robinson begins her film by introducing Dr. William Moulton Marston (Luke Evans), a dynamic psychology professor at Radcliffe, where he expounds on his DISC theory (that human behavior is determined by the concepts of dominance, inducement, submission and compliance) to his classes of alternately enthralled and embarrassed young women. Sitting on the windowsill as he lectures is his tart-tongued, brilliant wife and collaborator Elizabeth (Rebecca Hall), who’s fuming about Harvard’s exclusionary policies toward women in its PhD program, which have stymied her career.

The Marstons are working on perfecting a lie detecting machine, but thus far have not succeeded. That changes when one of the new students in William’s course, Olive Byrne (Bella Heathcote), becomes a teacher’s aide to the professor. After some initial resistance from Elizabeth, they effectively become a threesome in every respect—Olive’s input actually enables the lie detector’s completion—and their unusual relationship leads not only to Olive’s breakup with her morally uptight fiancé Brant (Chris Conroy) but Marston’s dismissal from the faculty.

The trio doesn’t let that tear them apart, since each of them loves both of the others (and Olive becomes pregnant to boot). They move into a suburban house together, and must try to make do financially since Marston, an idealist, has given up the patent to the lie detector. Elizabeth takes a demeaning job as a secretary, while Marston’s attempts at writing pop psychology books fail. Happily, he finds his way into a Greenwich Village S&M shop where elegant Charles Guyette (JJ Feild), known as the G-string King, presides. He introduces the three to bondage role-playing. Olive also tries on an outfit that strongly resembles the costume Wonder Woman would later wear.

In fact, it is the experience at Guyette’s store that gives Marston the idea for the character, whom he sees as an expression of his proto-feminist ideals and psychological theories. He sells the idea, without much difficulty, to Max Gaines (Oliver Platt), identified as “the man who discovered Superman,” and the superheroine’s first appearance in Sensation Comics shortly follows. She’s quickly a smashing success.

That means relatively good times for William, Elizabeth and Olive, whose idyllic existence is, however, threatened when a neighbor unwittingly stumbles in on one of their more explicit role-playing games and intolerance against their unusual lifestyle—and more importantly, against their children—arises. A threat also appears to Wonder Woman’s well-being in the form of child study expert and moral crusader Josette Frank (Connie Britton), who protests against the sexual undercurrents in Marston’s stories, particularly in terms of bondage and submission. (Excerpts from a hearing in which Marston is questioned by Frank act as linking devices throughout the movie.)

As anyone who cares to read Lepore will soon discover, Robinson’s script—which does not cite the Harvard historian as a source—not only simplifies things considerably (as one would expect of a docu-drama), but paints them in a much more agreeable light. (Marston’s wife did not take to the ménage a trois quite so willingly as portrayed here, for example.) But Robinson is fashioning a fable about female empowerment and sexual liberation, not a documentary, and as such her version, while hardly high-level history, does what it sets out to do, which is to make viewers feel good about the free-spirited, progressive folks they see on the screen, acting (and sounding) like Noel Coward characters on a roll.

Of the actors, Hall is especially successful pulling that off, spouting her lines—liberally laced with foul language—with the practiced skill of a British raconteur. Evans and Heathcote are more like second and third bananas, but sell their characters reasonably well. Of the supporting cast, Feild and Platt come off best, treating their scenes like the comic riffs they are. The period detail in Carl Sprague’s production design and Donna Maloney’s costumes is a winning addition, and Bryce Fortner’s cinematography captures it all in glistening widescreen images.

“Professor Marston” does not deserve to be called wondrous, but it’s an amiable stroll through some interesting territory. And it points to one very real fact: while Marvel might have taken the lead in modern superhero movie franchises, the DC lineup has by far the best backstories. The unhappy fate of TV’s Superman George Reeves has already been told in “Hollywoodland,” and now Marston’s tale has been dealt with. Perhaps we will soon see Jerry Siegel and Joe Shuster, Superman’s creators, brought to the screen. Their treatment at the hands of DC could make for a good legal thriller.