\”This is gonna be fun!\”So declares Arthur Curry, the titular Aquaman played by Jason Momoa, at the end of his movie, after (spoilers? I guess?) he defeats the bad guys, saves the world, and gets the girl.The phrase is a good thesis statement for Aquaman, a movie that wants desperately to be a good time. After the so-called DC Extended Universe debuted with 2013\’s Man of Steel, most of the entries have been marked by the heavy hand of director/producer Zack Snyder. As a result, the DCEU often felt like the emo kid brother to the more critically and commercially successful Marvel Cinematic Universe, confounding darkness for depth and cynicism for smarts. After the disastrous Batman vs. Superman: Dawn of Justice, which featured the world\’s finest superheroes slugging it out in a dirty bathroom, a few DCEU entries tried for a course correction, but only the excellent Wonder Woman managed to capitalize on the characters\’ potential.Despite the unlikely hero at the center (the whole \”talks to fish\” thing has been a meme as long as the internet has been around), Aquaman has a lot of good stuff going for it. It\’s got James Wan at the helm, whose work on the Conjuring and Insidious movies has shown him to be a first class genre director. Teaming with cinematographer Don Burgess and production designer Bill Brzeski, Wan creates an Atlantis that\’s an extravagant CG update to The Little Mermaid\’s \”Under the Sea\” sequence: vibrant, strange, and engrossing.Even better, Wan\’s assembled a great cast, including overqualified ringers in Willem Dafoe and Nicole Kidman as the hero\’s mentor Vulko and mother Atlanna, as well as newcomer Yahya Abdul-Mateen II and veteran Patrick Wilson as villains Black Manta and Ocean Master. Heck, even Dolph Lundgren shows up for his second appearance in a 2018 Hollywood blockbuster.A hulk with charisma as massive as his biceps, Jason Momoa\’s the near-perfect choice for this version of the character, a guy whose heroism seems motivated more by a love of adventure than some tragic backstory. He dives into danger with a smirk and a cheer, repeatedly assuring us that he\’s having the time of his life.All of this potential gets paid off in the movie\’s first action sequence, in which Aquaman thwarts Black Manta\’s hijacking of a Russian submarine. Wan\’s camera whips around the sub as Momoa pounds his way through like a a professional wrestler, stopping only to catch a goofy expression from a KO\’d bad guy or Aquaman\’s giddy grin. We even get a legitimately interesting motivation to Black Manta\’s villainy, in the film\’s only effective dramatic scene. It\’s a lot of fun, something you could only get in an Aquaman flick.And then the movie has to go and scuttle it.Curry\’s mixed half-Atlantan/half-human heritage becomes the focus, as does his half-brother Ocean Master, who leads a coup to unite the fractured kingdoms of Atlantis and launch an attack on the surface world, forcing the filmmakers to unleash a flood of backstory. The sea magician Mera (Amber Heard) enters the story, using her abilities of water control and exposition recitation to compel Arthur into battle against Ocean Master. A fetch quest ensues, turning the movie into a lesser Tomb Raider, as Arthur and Mera search for clues (but find love!) in places like Sicily and the Sahara.Absolutely none of it works. Momoa and Heard have zero chemistry together, but their romance is far from the only point in which the plot mechanics creak along like the gears of an ancient stone machine. The movie needs every second of its 2 hour and 23 minute run time to spell out its familiar quest narrative and its \”we\’re all alike, even if we\’re different\” theme, using clunky shortcuts to lend gravitas to random scenes.A staff-spining move Vulko teaches Arthur in an insignificant early moment becomes a key part of the climax when the movie wants to hammer an emotional connection between the two. Puzzles get instantly and improbably solved by Mera\’s magic powers, powers we\’re supposed to forget when she\’s being pursued by invaders in suits filled with water.And even if someone happened to be enjoying the story, Arthur repeatedly makes cracks at its expense (example: Aquaman summarizes the revelation of a key piece of lore as \”Something something trident\”). These jokes are supposed to make Arthur cool and likable, and they kind of do, but they also underscore the perfunctory plot.Worse still, the movie squanders its cast and setting by playing to the actors\’ weaknesses. Wilson has done great work as a regular guy in fantastic situations, but he comes off bored as a would-be underwater emperor. The narrative keeps draining the excitement from Momoa\’s performance by asking him to brood over the Atlantans\’ mistreatment of his mother and dulls his visceral physicality by forcing him to participate in fake, floaty underwater battles. Heard comes off worst of all, given reams of exposition that she simply cannot make interesting, let alone.Of course, the movie doesn\’t want to be regal as much as it wants to be fun, fully embracing high-adventure tropes from everything from Indiana Jones to decades of DC comics.But all of it, even an octopus playing the drums, feels like a collection of gestures more than something actually delightful. The movie tells us that it\’s fun, and we really want to believe it, but the good times quickly drown in an ocean of mediocrity.