Ken and Maria are an unmarried but committed couple who are in a long-running and stable relationship. Perhaps things feel a little too stable to them, however, because they have invited two other couples to join them at a secluded cabin for a weekend of sexual experimentation with each other.
Jason and Diana are the first couple invited. They've also been together for a long time, but their relationship is much more volatile. They've split up several times, and Jason throws a curveball into the plan when he shows up with Ginny, a sexually adventurous young woman he's known for only a week. It seems he and Diana have split up again, though he blithely reassures Ken and Maria that Ginny "is into this stuff". The idea that introducing a stranger might throw everyone else off balance seems not to have occurred to him. But then, as we'll learn in the course of the film "thinking things through" is not Jason's strong point.
The final pairing are Catherine and Mark. They're the only couple who are married, but they're also the most dysfunctional: we see that from their very first interaction, when Catherine collects Mark to drive out to the cabin. Mark is also the most reluctant about the whole proceeding, to the point where it's not really clear why he agreed to come at all. Based on his other character traits, possibly just because be kept putting off the argument he knew it would cause. Catherine, you see, is deeply unhappy with their non-existent sex life and desperately wants something to happen this weekend.
All these issues collide in this low budget Canadian offering, which draws both drama and humour from said collision. It's a generally engaging film, and manages to make the characters mostly sympathetic even at the time they're being jerks (and pretty much everyone acts like a jerk at some point in this).
I enjoyed The Cabin Movie, and if you're interested in stories about how sex, friendship and romance interact, it's worth a look. It's not flawless but it eschews the most common developments you might expect from such fare. In particular I liked that Ginny doesn't fall into the 'manic pixie dream girl' schtick as much as it first appears she will.