Could the 21st century thrust polyamory back into the mainstream? Some scholars have theorized that way back when—not just in the freaky-deaky 1960s, but in humanity’s hunter-gatherer days—people relied on group love for survival and, well, entertainment. (It’s a long story. Check out Christopher Ryan and Cacilda Jethá’s Sex at Dawn.) If future generations continue to cling to monogamy like a Talbot’s sweater set fresh out of the dryer, it would only be because not enough of them have seen Tom Twyker’s Drei (Three). The German director’s graceful, sensual film takes a couple, Simon (Sebastian Schipper) and Hanna (Sophie Rois), whose 20-year relationship is gradually deflating. Then Hanna meets Adam (Devid Striesow), a rosy-cheeked stem-cell scientist at a hearing and, within days, is tumbling around in bed with him. Meanwhile, Simon undergoes surgery for testicular cancer, and meets Adam at the local aquatic center. He’s never been with a man before, but something about his new swimming partner is insanely alluring. Hot locker-room action ensues. (Something tells me Twyker knows his gay porn.) Simon and Hanna don’t seem particularly remorseful about their affairs—in fact, their mutual love for Adam fuels a renewed lust for each other. They’re thriving; they even begin to smell different. But they still have no idea they’re in love with the same man until an encounter reveals their improprieties. If what happens next doesn’t force us to reconsider our society’s obsession with monogamy, we deserve every suffocating, single-partner relationship we plod through evermore. Long live the throuple!

The film shows at 6:30 p.m. at the Goethe-Institut as part of its “Seize the Day: Outstanding German Films You Might Have Missed” series. $7 for nonmembers. goethe.de/washington. (202) 289-1200.