Rating: 3.5 stars

Aside from boutique horror fans, most filmgoers haven’t heard of Justin Benson and Aaron Moorhead, despite raves for the filmmakers’ 2014 breakout, the monster movie “Spring,” and a spot on Variety’s 2015 “10 Directors to Watch” list. But with the duo’s follow-up “The Endless,” they are becoming increasingly difficult to ignore. The new movie — a sci-fi freakout that, like “Spring,” includes an “it,” but one that’s far less easy to define — is spooky, funny, touching and very, very well made.

Benson and Moorhead, who met as interns working for Ridley Scott’s production company, share multiple credits here — co-directing, acting and producing — with Benson handling writing duty and Moorhead the cinematography. In “The Endless,” they play brothers who, after receiving a videotape from a member of the UFO death cult from which they fled 10 years ago, decide, on a whim, to revisit the idyllic rural compound Camp Arcadia.

It’s not as unlikely as it sounds. Aaron, the younger sibling (played by Moorhead), harbors mostly fond memories of his communelike childhood home, where they were raised after a car accident that killed their single mother. Justin (played by Benson), indulges his little bro, although he harbors deep misgivings about what he calls the “culty” aspects of the group, which, as he has explained it to Aaron, include mandatory castration.

Upon arrival, that and other lurid idiosyncrasies seem unlikely. Its members are exceptionally chill — thanks, perhaps, to the so-called “flower” that many of them smoke — and exceptionally youthful. Most of the Arcadians, who should be at least in their 40s by now, seem far younger. But this is California.

Aaron, who has grown sick of his dead-end life back in the city, where he and Justin clean houses for a living, subsisting on ramen noodles, decides that he might like to stay where the air is clean, the food is healthy and the living is easy. (The Arcadians, who resemble the members of an especially earnest vegan food co-op, make money by producing and selling craft beer.)

But all is not as it seems. The group’s de facto leader (Tate Ellington) speaks in cryptic riddles — spouting things like “Nothing here ends,” tinkering on an obscure mathematical formula on a chalkboard and alluding to something called the coming “Ascension” — which sounds suspiciously like what the real-life Heaven’s Gate people spoke about before 39 of them were found dead from mass suicide in 1997.

Other weird phenomena include strange behavior by birds and at least two moons.

If all this sounds pretty preposterous, it is. But Benson’s screenplay and the performances by the convincing ensemble cast, several of whom are given dialogue that echoes — and, more important, defuses — our own natural skepticism, help to sell what in other hands would be irredeemably silly. It also helps that the script includes lots of subversive comedy, especially in the bizarrely hilarious confrontation between Justin and a camp neighbor known as “S----y Carl” (James Jordan) that reveals the true nature of Arcadia’s secrets.

As science fiction, “The Endless” is never less than engrossing, but it also grounds itself in recognizable human drama, with a subplot focusing on Justin and Aaron’s sibling rivalry and love. It raises provocative questions: about eternity, free will and the paradox of a paradise that may or may not be a prison. That’s a lot to ask of a monster movie — or whatever you want to call “The Endless” — but Benson and Moorhead deliver.

Unrated. At the Angelika Pop-Up at Union Market. Contains crude language, drug use and some violent imagery. 112 minutes.