This photo is from the official trailer for “Happily,” distributed by Saban Films.

Tom (Joel McHale, “Becky”) and Janet (Kerry Bishé, “The Evening Hour”) can’t stop doing the hanky-panky. They boink in the bathroom. They get sexy on the stairs. Their marital bliss knows no bounds. And they’ve been married for 14 years.

This is the central premise of “Happily,” the directorial feature BenDavid Grabinski (“Cost of Living”) — what if the honeymoon phase stayed ever so honeyed? For Tom and Janet, this means a couple of things.

One, they alienate all their friends. Their libidinal zeal and unshakable puppy love make everyone else look downright ready for divorce. The second thing is that their freakish affection is, in fact, freakish — or at least that’s what a mysterious man in a suit (Stephen Root, “Home”) says that operates on an appropriately ominous “higher level of authority” than the government. The solution to their monstrously paradisal union? A couple of syringes of lime-green fluid.

Cue a little panic, throw in a blunt object and boom — a mysterious man in a suit is a mysterious dead man in a suit. After receiving an invitation to a swanky Airbnb getaway out of town with their disenchanted friends, they’re all too quick to accept, social cohesion be damned.

Marital harmony is the film’s primary quarry. Does love spoil like milk or should it age like wine? Can you ever know everything there is to know about a person? These are big questions with, frankly, obvious answers, but the investigation of them through a dark comedy-thriller is a worthy and entertaining one. McHale and Bishé perform admirably as the ever-in-love couple with a particularly delightful introduction that has them courting each other as if they were strangers at a party, with scintillating close-ups and a darkly neon color scheme. The murder of the mysterious suited man seeking to “correct” the “mistake” of their perpetually burning love adds just enough movie-magic juice to notch the whole deal up a level, and the spacious, decked-out Airbnb provides a stylish setting in which the plot unravels.

Unfortunately, the film takes its premise and runs nowhere fast. 

The film is something of a mystery movie, and, like any good mystery, it has its windy twists and turns. Baits-and-switches. Red herrings. And more red herrings. In fact, a whole school of red herrings. The film is relentless in setting up little reversals for comedic effect. One character ominously informs another that “I have something in the trunk that you’ll be very interested in,” only for it to be … pizza. Another character threateningly lurking in a dark corner is there because she’s ridiculously high. These little switches are never confusing but they pile on false start after false start so that soon it’s the film that cried wolf. 

The listlessness of the second half is also in large part because of an uncharismatic cast of characters. Paul Scheer (“Archenemy”), Natalie Morales (“Language Lessons”), Shannon Woodward (“Ode to Joy”) and a smattering of spouses round out the ensemble of alienated friends. They are all delightful actors, but they are also woefully underserved by vapid characters and zingers that fall flat. Scheer’s Val is a chef that used to prank people. His wife Karen (Natalie Zea, “The F**k it List”) really wants to sleep with Tom. This is about as much characterization that Val and Karen receive, and they have more screen time than the six (six!) other characters. A group therapy session near the tail-end of the flick in which the gang divulges their guts in comic fashion is a whole lot of “meh.” Without knowing who these characters were in the first place, the twists and reveals of salacious secrets don’t have much effect beyond a half-hearted chortle.

The first act-and-a-half are enormously intriguing, and “Happily” has a novel-enough premise that will make the film worthwhile for anyone that masochistically enjoys ambiguous oddities. But unlike Tom and Janet’s love for each other, the film’s 95-minute runtime cannot escape the law of diminishing returns.

Daily Arts Writer Jacob Lusk can be reached at luskja@umich.edu.