It might come as a relief, I guess, when you reach the bloody end of Hafsteinn Gunnar Sigurðsson’s slow-boil neighbor-versus-neighbor comedy-gone-wrong Under the Tree and realize oh, good, it’s not just America’s well-to-do white retirees who have lost their damn minds. Sigurðsson’s film concerns, among its twin dysfunction plots, two pairs of suburbanites living in what any reasonable person might assume is bliss: elegant, comfortable, architecturally progressive bungalows in a wooded suburb of Reykjavík, Iceland. But it’s the fate and function of big-screen suburbia to be forever exposed as a hotbed of some vice or another.

Here the vice is familiar but still worth satirizing. Turns out, everyone’s deeply suspicious and resentful of everyone else, always assuming the worst. So much so that, in just 89 minutes, Sigurðsson builds from one husband requesting that the other prune a backyard tree to an eruption of vicious violence. The first scenes are hilarious, all sharp surprises and adeptly staged physical comedy. But then the story turns, the way that milk does, curdling into tragedy. You might find the concoction unpalatable; I laughed and cringed and caught myself once in a while holding my breath, dreading what was next. When’s the last time you saw a narrative feature with a genre that was so shrewdly unfixed that you genuinely had no idea what would — or could — happen next?

It’s the kind of movie where, just seconds after you smile at the entrance of a grandly fluffed housecat, you immediately start bracing for its possible demise. Kitty goes missing, of course, as tensions escalate between the families. But Sigurðsson maintains exquisite control of our expectations, goosing and dashing them. Just when I thought I had gotten ahead of the plot, and that Baldvin (Sigurður Sigurjónsson), starting the car and backing out of the driveway, would thump right over his poor cat, Sigurðsson offers something else entirely. His characters, like his script, go big early, before you might expect them to, speaking terrible truths or dire calumnies. Then they surprise again, in the later scenes, by exhibiting occasional wisdom and tenderness. Or not. (The performances are convincing, each actor committed to a gently heightened naturalism no matter how outlandish the film’s reality gets.)

Sigurðsson is toying with us, as that cat might with its prey. It’s not especially provocative to suggest that people who behave with decency toward their loved ones can quickly be moved to behave abominably toward everyone else. Somewhat more challenging is the film’s counterpoint storyline, which finds a young married couple, one the son of a set of the dueling neighbors, breaking up in volatile fashion. In the first scene, one of Sigurðsson’s best, Agnes (Lára Jóhanna Jónsdóttir) walks in on Alti (Steinþór Hróar Steinþórsson) watching porn on his laptop. He slams the computer shut, but then the video leaps to the monitor on his desktop workstation. And as you might be laughing at that, Agnes notices that one of the participants in the sex is Alti — and the other a woman they both know.

Soon, Alti is crashing with his parents, the voice of reason in their feud with the neighbors; he keeps noting, with a son’s disinterested annoyance, that his folks are making too much of everything. But he’s making too much of everything, too, behaving threateningly toward Agnes at work, chasing her in his car, showing up at their daughter’s kindergarten and taking the tyke on a picnic in the IKEA parking lot. The most surprising thing in this scathing satiric bloodbath: that the character who at first seems the film’s vilest is the one who eventually faces and reckons with himself.

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