Before we get whisked away to Margaritaville, Wan and the film's screenwriters have a bit of rote backstory to establish. The movie opens with a Maine lighthouse keeper (played by Temuera Morrison, of Jango Fett fame) discovering the body of an Atlantean princess (Nicole Kidman) washed ashore; the two fall in love in quick fashion, bonding over their mutual appreciation of fish and passion for staring out at the waves, and they soon have a son named Arthur, who holds the potential to bridge the divide between their two respective worlds. After his mother is captured and taken back to the sea, the young man learns that with great power, like talking to sharks at the local aquarium, comes great responsibility from his mentor, Nuidis Vulko (a bizarrely understated Willem Dafoe).
Baby Arthur eventually grows into the hulking Momoa, who gets greeted with a guitar riff almost every time he flips his long, constantly wet locks. He enters the movie by taking out a technologically advanced team of pirates led by Black Manta (Yahya Abdul-Mateen II of The Get Down), a villain who will likely become more important in future sequels but mostly spends his screen time here tinkering with his gear. There's a lot going on in the movie's 142 minutes.
The more important bad guy in the film is Arthur's half-brother Orm, played by the aforementioned Patrick Wilson. A stuck-up product of Atlantis's ruling warrior class, Orm wants to control the many factions of the ocean, which include a race of large talking crabs, and declare war on the "surface-dwellers," who he believes are destroying the planet with pollution. (Like the best villains, Orm has a good point, but the movie isn't terribly interested in exploring the nuances of his environmental imperialism.) Despite having little interest in becoming a king, Arthur is pressured by would-be queen Mera (Heard) to challenge Orm and wrest control of the underwater world. As you'd imagine, this larger quest narrative is mostly an excuse to string together wondrous special-effect showcases, like a Top Gun-style dogfight in an aqua-pod, and explosion-filled action set pieces, like a thrilling parkour-and-laser-blasters chase through a beautiful Italian villa. At one point, dinosaurs show up and they almost don't even register.