Of course young director Charlie McDowell could get his first feature-length film made. He’s got mega-connections as the son of veteran actors Malcolm McDowell and Mary Steenburgen, and his stepdad is Ted Danson. Nepotism-fueled mediocrity is rampant in the movie biz, which is why it’s such a pleasant surprise when the result is instead a rewarding, odd little film like “The One I Love,” distinguished by an original premise, quiet chutzpah, realistic dialogue and masterful mood-setting.

Ethan (Mark Duplass) and Sophie (Elisabeth Moss) go to couples therapy in an attempt to repair a broken trust and get back to where they used to be — jokey, affectionate, having sex. Their therapist (Danson, in a cameo) suggests an isolated nearby retreat where, he says, clients learn to “reset the reset button.”

Upon arriving at the artsy, idyllic abode, where they’re the only people around for miles (or are they?) Ethan thumbs through glowing reports of renewed amour in the guest book. The pair have a romantic dinner, and all systems are go for a rousing makeup session in bed, until Sophie decides to check out the adjacent guesthouse, where she discovers that maybe they’re not alone after all. Well, not exactly.

Here’s where it gets challenging to avoid giving too much away. A huge part of this movie’s magic is hitching a ride on this troubled couple’s seriously disconcerting mind trip rather than knowing in advance what they’re about to go through. And it’s not something you’d see coming. Let’s just say, in Ethan’s words, that they’re smack in the middle of “a cosmic aberration,” or as Sophie speculates, “another dimension, a different plane of existence.”

No one can accuse screenwriter Justin Lader, also making his feature-film debut, of being derivative. And original music by Danny Bensi and Saunder Jurriaans envelops the viewer in moods by turns eerie and quirkily comforting. But much of the credit for making this gamble work goes to its two stars and the shifting dynamic between them.

Moss, best known as copywriting ladder climber Peggy Olson on “Mad Men” and a tough but vulnerable police detective in Jane Campion’s miniseries “Top of the Lake,” further stretches her range here, bringing subtle layers to Sophie’s emotions without speaking a word. Duplass, mostly known as an affable boyfriend and/or lovable jerk (“Safety Not Guaranteed,” “The Mindy Project”) shoulders a lot more pressure to carry a story than he has before, and pulls it off.

Is it better to be happy with a perfect, but falsely so, vision of the man or woman we married, or to face truths that will sometimes make us miserable? With one more tiny twist in the end for emphasis, “The One I Love” is a weird but gutsy exploration of what happens when we alternately idealize and find fault with our romantic partners.