"OK, first of all, it's like he never flossed."

What we know at the beginning of “Annihilation” is that a lighthouse was struck by a projectile that fell from the sky (seldom a good omen) and that Natalie Portman, in hospital clothes, is being interviewed by three men in hazmat suits who want to know what happened “in there.”

What follows is a gripping, uneasy sci-fi horror story in the tradition of “Aliens” or “Predator” (though not “Alien vs. Predator,” strangely), thoroughly engrossing, occasionally gross, with a bit of Kubrickian trippiness thrown in for good measure. It’s the second film directed by screenwriter Alex Garland, following “Ex Machina,” and while it’s an adaptation of a novel (by Jeff VanderMeer), it has some of the same themes as original Garland scripts like “28 Days Later” and “Sunshine”: a small group of people exploring the unknown, unsure what terrors await them (but probably doomed).

Portman plays Lena, a Johns Hopkins biologist who spent several years in the military. That’s where she met her husband, Kane (Oscar Isaac), who went on a secret mission a year ago and never returned. But wouldn’t you know it, at the very moment Lena is listening to sad Crosby, Stills & Nash songs and repainting her and Kane’s bedroom to symbolize her decision to move on, he shows up, seemingly none the worse for wear. What happened? Where’s he been? He doesn’t really know. He’s … not the same.

Where he was, it turns out, was with a team investigating that lighthouse. Or trying to investigate it. The extra-terrestrial object that hit it created a shimmering bubble that has expanded to cover many miles of land and shows no signs of stopping. Simply called the Shimmer by the military’s top scientists, it’s not clear what effect it has on people because so far, except for Kane, nobody who has entered the Shimmer has returned. Something in there is killing every team they send, driving them crazy so they kill each other, or a combination of the two.

Whatever it is, Lena wants to know, for reasons both scientific and personal. She joins four other science ladies on one more excursion into the Shimmer: Dr. Ventress (Jennifer Jason Leigh), a passionless psychologist; Josie (Tessa Thompson), a kindly physicist; Cass (Tuva Novotny), a European anthropologist; and Anya (Gina Rodriguez), a fiery paramedic. There’s a certain fatalism in embarking on a mission that almost no one has ever come back from, and the movie is overshadowed by a sense of doom from this point forward.

The Shimmer has overtaken a chunk of what must be Florida or thereabouts, covering marshes and swamps and at least one very large alligator. The scientists find that their compasses and communications devices don’t work (no surprise), and that the alien substance alters — that is to say, mutates — whatever plant and animal DNA it comes in contact with. Simply being IN the Shimmer, whether you touch anything or not, seems to be enough to do the trick, a fact that dawns on our quintet gradually.

I won’t tell you anything else that happens in the Shimmer. (It’s like Las Vegas in that respect.) Having introduced us to a world with limitless possibilities, Garland lets the madness creep up on us: paranoia here, dangerous creatures there, always a sense of disorientation. Lena is the only character whose thoughts we’re privy to, and we see flashbacks to her and Kane before the mission that shed some light on their motivations. The other women, though not as well developed as characters, each add flavor to Garland’s tense and frequently terrifying concoction, even if some of their doom-laden conversations don’t amount to anything more than dreadful garnish.

A few elements, like the Shimmer’s effect on one’s sense of time, are announced but unexplored, though I note that the “Annihilation” novel is the first in a trilogy. (I also note, however, just by reading the Wikipedia summary, that the movie deviates significantly from the book.) The finale fits the tone of the rest of the film — things weren’t cheerful before; why would they start now? — in a way that will feel cold and perhaps unsatisfying to some viewers, but I ate it up, reveling in the unsettling events and their eerie implications.

Crooked Marquee

B+ (1 hr., 55 min.; R, some harsh profanity, a little sexuality, some graphic violence and gore.)