Ethan (Mark Duplass) and Sophie (Elisabeth Moss) are seeing a therapist (Ted Danson - white-haired but still handsome) because their marriage is on the rocks. Their therapist encourages them to go to a particular retreat, saying every couple who's gone has come back happy, rejuvenated, in love again. So off they go. They're alone at the retreat (in fact I've now introduced the entire cast - no joke, no one else). But every time one of them goes from the main house into the guest house, really weird shit happens.
I can't say much more without giving away a lot of that happens in the movie. But I'm going to add that I think this should be classified as Science Fiction, because by the time we're done things have happened that simply aren't possible in the modern world as we know it: it posits technology that doesn't yet exist.
Setting that aside, I had a fairly ambivalent reaction to the movie as a whole. The good news is that you never know where the hell it's going, and yet it's still well constructed ... the bad news is that I didn't much like the resulting product, even though it was kind of intriguing. You do get nicely skewered at the end.