J.J. Abrams has a maddening tendency to oscillate between almost transcendent poignancy and superficial enigmas. Perhaps best known for his TV show “Lost,” his monster movie fake-out “Cloverfield” and his successful directing in 2009’s “Star Trek” reboot, Abrams has established himself as the one of the more credible sci-fi-leaning auteurs. But at his very best, it’s not at all about a mysterious island or intergalactic space wars — it’s about the characters that inhabit these worlds.

You wouldn’t find a better-sketched character than in a J.J. Abrams production. While we always manage to get an exceptional narrative and emotional payoff from his characters, the science fiction story he sets them in often gets muddled to the point of futility. “Super 8,” his first movie to both write and direct, is his fluctuating brilliance writ large.

It’s also like a giant piece of fan mail: Steven Spielberg, Abrams’ childhood idol, serves as a producer on the film, and his influence is apparent throughout. Many of the sequences in “Super 8” are Abrams trying to create the kind of classic movie moments Spielberg has famously built his career around. While he earns a few great moments that stay with you, he stops short of Spielbergian brilliance, never quite eliciting that movie magic. Abrams’ next, granted it’s without someone like Spielberg’s guiding presence, will be his true test — to see what he can do other than pay tribute to his hero.

Set in the 1970s, the story is tied to Abrams’ childhood of shooting amateur films on 8-millimeter, or Super 8, cameras. Joe (newcomer Joel Courtney, who will have no trouble finding work after this star turn) works with his best friend Charles (Riley Griffiths) to make a movie to submit to a local film festival. Rounding up their group of friends, they convince the reclusive school beauty Alice (Elle Fanning, who proves again how better a performer she is than her sister Dakota) to play their lead actress. Together, they sneak away in the night to shoot at an out-of-the-way train station.

While shooting, they witness an explosive train wreck. After the crash, they hear something clobbering its way out of a train car. Before they can investigate, a military convoy swoops in, quickly taking over the kids’ small Ohio town with little explanation. From there, bizarre things begin happening: all the dogs in town run away, people go missing and electronics, from toasters to car engines, vanish without a trace.

Among the uninformed is Joe’s father and sheriff’s deputy, Jackson (Kyle Chandler, who wrings a terse, affecting performance with a small role), who is suspicious of the train crash and the military’s involvement. Joe and his father’s relationship has grown tense since his mother died in an accident — tenser by Joe’s flowering relationship with Alice, whose miscreant father has been a serious thorn in Jackson’s side.

Joe’s relationship with his father and Alice are the emotional core of “Super 8,” and with Abrams at the helm, they and all other character relationships flourish. These characters, so fully-realized, well-drawn and lifelike, are the most compelling aspect of the movie. Abrams’ script is wrought with tenderness, flashes of humor and a knowledge of how to get us to trust the characters’ backstories without hesitation.

The monster mystery, meanwhile, is less elegantly handled. As with any good mystery, the best part is the anticipation, not the reveal. Abrams knows this well and frames some genuinely tense scenes of that shadowy, monstrous figure lurking off-screen.

The final reveal is a self-fulfilling disappointment, but also beside the point. It doesn’t matter what came out of that train car as long as the buildup to finding out is gripping. Here, finding out the answer to this mystery is too formulaic and offensively lame. The story places these far more interesting characters into situations in which they can interact with one another, but the imbalance between story and character is too stark. You stop caring at all about the whole story before the third act and just wish these great characters could do something else.

Unlike the science fiction element of the movie, you can tell the characters were more carefully managed. It would never work without its cast of young actors. Made up mostly of newcomers, this is the most enjoyable group of movie children since “The Goonies.” Their camaraderie is infectious and strong enough to carry the bulk of the movie.

It’s difficult to know how much better, if at all, a movie “Super 8” would be if it hadn’t had the monster-thumping-in-the-shadows side to it. A movie about the emotional relationship ties between friends, much less children, is a hard movie to sell; harder to even get someone to put up the money to get it made. It’s one mystery more for Abrams.