Gal Gadot’s supporting role as Diana Prince in last year’s “Batman V. Superman: Dawn of Justice” served as an appetizer for the Amazonian superheroine’s full introduction into the big-screen DC Universe presided over by Zack Snyder. Now she becomes the main course, with her own big-budget summer blockbuster. She’s great, and her movie is comparatively good.

You can almost see the asterisk at the end of that sentence, though: there is a qualification. Yes, “Wonder Woman”—directed smoothly, and with some perceptible female sensitivity, by Patty Jenkins (“Monster”)—is better than the earlier pictures that have come out of Snyder’s Warner Brothers DC assembly line—“Man of Steel,” “Batman V. Superman: Dawn of Justice,” “Suicide Squad.” It also far outshines the cheesy seventies TV series with Lynda Carter. By any other standard, though, it’s only fair.

It does, however, have some substantial virtues. Most important are the stars. Gadot proves an excellent choice to fill Wonder Woman’s new boots far better than either Henry Cavill’s Superman or Ben Affleck’s Batman. She’s beautiful, of course, but also able to convincingly manage the action requirements while projecting a sense of naivety that can quickly change to an expression of righteous anger when needed. In short, she capably handles every one of the role’s requirements, and does so with elegance and a hint of mystique.

Chris Pine is equally fine as Steve Trevor, the first man Diana ever sets eyes on and her inevitable romantic interest. The script by Allan Heinberg has to go to unlikely lengths to situate an American into its revisionist historical template, but Pine’s combination of boyish bravado and seriousness makes one swallow it without complaint. Trevor is not just a lunkheaded piece of beefcake in distress here, but a thoughtful guy who must symbolize both mankind’s frailties as well as its potential for nobility—and Pine actually manages to pull that off, a task no less challenging than the one Gadot faced. It helps that the two stars have chemistry, too—something increasingly rare with stars in American films.

Another point that works in the picture’s favor, to some extent at least, is one that might irritate some comic fans—the change in the timing of Diana’s departure from Paradise Island (or Themyscira, to be more precise). The script draws elements from various origin mythologies that writers have concocted over the decades: Diana (played as a tyke by young Lilly Aspell and then as a teen by Emily Carey) has been sculpted from clay by her Amazon mother Hippolyta (Connie Nielsen), given life by the Amazons’ protector Zeus (whom they had once helped to defeat the rebellious war god Ares), and then trained as a warrior by her aunt Antiope (Robin Wright). They live in splendid isolation on the island Zeus has given them, champions of peace through strength—until one day a plane crashes in the nearby sea and its pilot, Trevor, is rescued by Diana and brought to the island. Diana will leave with him and become Wonder Woman in the outside world.

In all the traditional accounts, that happens in 1941 (the year of the comic’s creation), and her enemies are the Nazis. Heinberg, however, changes the time to late 1918, when World War I was reaching its end after more than four years of ghastly carnage. Trevor is now an American assigned to British intelligence who has infiltrated the command of German Erich von Ludendorff (Danny Huston, huffing and puffing like the Big Bad Wolf) and stolen the recipe book of the general’s chief chemical weapons scientist Dr. Poison (actually one of Wonder Woman’s original comic book villains, here played without much personality by Elena Ayala as a disfigured, half-masked figure), who has come up with a new gas that could alter the course of the war—or at least prolong it just when an armistice is imminent. Believing that Ares is behind the conflict and she is fated to confront him, Diana determines to accompany Steve to Europe.

The timeline alteration has one great strength: it places Diana in a heavy-handedly man’s world where women don’t yet even have the vote and are treated dismissively by the ruling establishment. Her reaction to the situation has a modern feminist edge, though one often treated comically, as when she is introduced to the uncomfortably confining women’s clothes of the period by Trevor’s motherly secretary Etta (Lucy Davis), or—when informed of a secretary’s duties, observes that back home they call that slavery. Those humorous notes, linking the same fish-out-of-water oddities that Steve has experienced on the island to Diana’s circumstances in the human world, give the narrative a welcome degree of lightness largely missing from the other DC movies. There are echoes of what the Marvel operation managed in the similar beats of the original “Captain America” and “Thor.”

Unfortunately, the time shift also robs the tale of a thoroughly odious villain—the Nazis—and so Heinberg has to refashion Ludendorff not only as a proto-Nazi (something that could be justified, one supposes, by the fact that the real general actually became a follower of Hitler before his death in 1937) but as an occasionally super-strong one (given amazing power by sniffing one of Dr. Poison’s formulas—a plot point that’s never explained). In reality, of course, Ludendorff pushed for the armistice, until he was sacked and became a proponent of continuing the war. Here, he stays in uniform and, after simply killing off all the other members of the command staff, undertakes to use his new poison gas to ensure that trench war will continue indefinitely.

It’s at this point that the picture begins to spin into genre cliché. With the help of an apparently enlightened, pro-armistice politician (David Thewlis), Steve and Diana assemble a small group of unconventional warriors—a Moroccan womanizer (Said Taghmaoui), a wild-eyed Scottish sharpshooter (Ewen Bremner) and a Native American smuggler (Eugene Brave Rock), who are obviously designed to bring some comic relief to the last act but don’t manage much of it—and are off to the Western Front, where Diana at last doffs her feminine garb and becomes Wonder Woman, striding across No Man’s Land to take charge of the German trenches and liberate the villages behind them. Her ambition, however, is to confront Ludendorff, whom she presumes to be Ares himself, and bring peace to man’s world once and for all.

The script cannot pretend, of course, that overcoming one malevolent being can pacify the globe, and its most compelling final message is that mankind itself must shoulder responsibility for conflict and overcome its worst instincts—and act heroically—to bring an end to it, a point made clear in an act of self-sacrifice of the sort that is becoming inevitable in films of this sort, but is carried off here more touchingly than usual. Still, the message is buried in a prolonged CGI confrontation between Wonder Woman and the forces of evil—not just Ludendorff but Ares himself. It’s the sort of explosion of lightning bolts, bursts of power and supernaturally propelled shards of metal and wood that is all too familiar—and frankly boring. Heinberg makes things worse by providing the ultimate villain with reams of taunting dialogue so formulaic that it seems across to catch in the throat of the actor delivering it; and attempts to give it a “period” twist only make it worse. You might have to suppress a laugh, for example, when the ray-shooting bad guy says egotistically to the fallen heroine, “Is that all you have to offer?” instead of the usual “Is that all you’ve got?”

Still, for roughly half of its nearly two-and-a-half-hour running time, “Wonder Woman” is a better-than-average superhero movie; a pity it falls off so precipitously in the last act. Apart from the editing of Martin Walsh, which should have been more active toward the close, and the visual effects supervised by Bill Westenhofer, which are unimpressive in that final confrontation though perfectly decent elsewhere, the movie is technically proficient. Though hobbled by the blandly brown color palette favored in the Snyder DC franchise, cinematographer Matthew Jensen’s lensing is topnotch, and the production design by Aline Bonetto and costumes by Lindy Hemming are first-rate (the frocks she provides for the “disguised” Diana are especially enticing, though the Wonder Woman outfit itself, deliberately avoiding the kitschy quality of Carter’s comic-based look, works well) Rupert Gregson-Williams’ score fulfills its function of adding dash to the visuals without being at all memorable.

One hopes that the quality uptick represented in Jenkins’ contribution to the expanding DC cinematic universe might continue. If the preview of the upcoming “Justice League” included as a teaser here is any indication, though, that hope threatens to be quickly dashed.