At its most inspired moments, “Aquaman” plunges straight into the deep end — like when a giant octopus commences an undersea gladiatorial contest by rapping its tentacles across a collection of oversized drums, or when someone offers the movie’s villain a weapon that “converts water into beams of energized plasma.”

I mean, if a movie is going to be this wacky, you really should give in. And I wanted to.

The problem with “Aquaman,” the latest attempt by the DC Comics faction to match their rivals at Marvel, is that it never picks which wave to surf. It offers an emotional origin story, but a glib, wisecracking hero. Its villain keeps a straight face throughout, but enormous action scenes culminate in stupid sight gags.

I guess director James Wan (a skilled hand at the “Insidious” and “Conjuring” franchises) means to ape comic-book style, where one panel is high melodrama and the next a goofy pun. You can get away with that in a comic, but it’s not so easy to sustain in a massive, effects-heavy, 143-minute blockbuster.

This uncertainty in tone affects Jason Momoa, whose previous appearances as Aquaman (recently in “Justice League”) suggested a humorous personality along with the six-pack abs. In this outing, Momoa looks lost between heroically saving the world or making feeble 007-style one-liners.

So: The origin story, because we can’t have enough of those. Lil’ Aquaman is the result of a loving liaison between a lighthouse keeper (Temuera Morrison) of the human world and a queen (Nicole Kidman, totally into it) of Atlantis. Mom is called back to her undersea realm, and her half-fish son spends years resisting the call of the water.

An Atlantean emissary, the mermaidy Mera (Amber Heard) seeks out Aquaman to settle a power struggle; seems Aquaman’s half-brother, King Orm (Patrick Wilson), is eager to wage war against the other undersea kingdoms.

Wilson’s sincerity works against the movie’s sense of fun; he’s the guy at the party who can’t be convinced to loosen up a little. Meanwhile, 1980s bad guy Dolph Lundgren (recently revived for “Creed II”) plays another fishy king, and sounds as though he regrets taking this gig.

It takes two hours for Mera and the Yoda-like Vulko (Willem Dafoe — wow, really?) to convince a reluctant Aquaman to find the magical hidden trident that can defeat evil. Oh, and there’s also Black Manta (Yahya Abdul-Mateen II), who possesses the aforementioned energized-plasma gun and has sworn vengeance against Aquaman.

Wan serves up huge battle scenes featuring thousands of swimming creatures (many, curiously, clad in battle armor), and the climax features some dazzling effects work. The underwater setting at least gives the movie some novelty. But frankly, there aren’t enough drumming octopi to make this movie memorable, an observation I thought I’d never have to make.

“Aquaman” (2½ stars)

The DC comics hero (Jason Momoa) gets his origin story while growing up to save the undersea world from his power-mad half-brother (Patrick Wilson). The film keeps wanting to get completely silly, but can’t decide whether to fully commit, and the style doesn’t wear well over 143 minutes of huge special-effects sequences.

Rating: PG-13, for violence

Opening: Alderwood, Alderwood Mall, Cinebarre Mountlake Terrace, Everett Stadium, Galaxy Monroe, Marysville, Stanwood Cinemas, Meridian, Oak Tree, Seattle 10, Thornton Place, Woodinville, Cascade Mall