Natalie Portman soars in “Lucy in the Sky,” as an astronaut reeling from the rapture of recent spaceflight. It’s another fine turn for a terrific actress, even if the film itself has a tendency to bump its tail on terra firma.

Portman plays the unfortunately named Lucy Cola, whose recent trip on the Space Shuttle was the culmination of a tightly wound career plan. Raised to excel by a demanding grandmother (Ellen Burstyn in good form), Lucy is at loose ends after she falls back to Earth.

She’s got a supportive husband (Dan Stevens) who also works for NASA in Houston, and her bosses (including Colman Domingo from “If Beale Street Could Talk”) appreciate her super-disciplined approach to training.

But Lucy can’t get space out of her head. It’s a hard act to follow. Focused on getting a spot on the next available shuttle mission, she starts making bad decisions.

These include an affair with a roguish astronaut (Jon Hamm), and rivalry with a new recruit (Zazie Beetz, the woman down the hall in “Joker”). Eventually things become unstable enough that she undertakes a breathless cross-country road trip in order to confront her perceived enemies.

If that last part sounds familiar, it’s because “Lucy in the Sky” is loosely based on a true story that happened in 2007. An astronaut, caught in a romantic triangle, drove for days to face down her rival; one unforgettable detail from the story was that the astronaut allegedly wore a diaper in order to minimize her travel time (a detail left out of the film, for which I give thanks).

“Lucy in the Sky” takes a serious look at this tabloid tale. Director Noah Hawley (from the TV “Fargo”) tries to summon up the poetry of Lucy’s journey, from her bliss in space to her dreamy distraction on the ground.

Portman is splendid. Although a tiny person, Portman’s swagger makes you believe she could easily fill an astronaut’s boots, and she’s good at showing how Lucy’s take-charge organizational skills might be used for unbalanced ends.

I also like Hamm’s turn as a womanizing spaceman. He’s got a great monologue where he tries to explain to Lucy how trivial their affair is, and how small their place in the universe.

In fact, a lot of “Lucy on the Sky” is compelling: the way Polly Morgan’s lush photography changes shape to suggest the narrowing of Lucy’s world, the training rituals at NASA, the courting habits of people looking for a little danger.

Eventually, though, the movie piles too much on its plate. Some labored symbolic action with caterpillars and butterflies, plus a bit of off-subject male chauvinism, feel heavy-handed.

The bad title is typical of the film’s tendency to push too hard. Her name’s Lucy, and she went to space — yes, we get the idea. There’s no reason to drag the Beatles into it.

“Lucy in the Sky” (2½ stars)

Natalie Portman soars as an astronaut whose post-spaceflight life is marred by bad choices. The movie is often compelling, but director Noah Hawley has a tendency to push his many ideas just a little too hard. With Jon Hamm, Zazie Beetz.

Rating: R, for language, subject matter

Showing: Meridian and Seattle 10