It’s the stock answer that many a happy long-term couple has given prying friends and relatives to explain why they haven’t married: “We don’t need a piece of paper to prove our love.” True enough. What can official documents tell you of something as wily and elusive as human desire? Is a band of gold a safeguard against a change of heart? Of course not, yet millions want it anyway, a ratification of feelings that might otherwise seem slippery or intangible from the outside. In “Fingernails,” a quietly searching and yearning science-fiction romance from Greek director Christos Nikou, the piece of paper in question isn’t a marriage certificate but a printed test result: a mathematical, machine-determined declaration that you and your partner are fully in love.

Sounds ludicrous, sure. But with a sly wit that doesn’t preclude its honest emotional intelligence, “Fingernails” invites us to consider whether such a concept is any sillier than the validating systems and symbols we devise — from marital contracts to algorithmic dating apps — to fix our relationships in place, or to make finite our search for another half. The longer its ideas gnaw at us, the less alternate its universe seems. Set either in a cozily low-tech future or a recent parallel past, where phones are still corded and car windows are manually wound, the disarming world-as-we-nearly-know-it air of Nikou’s fantasy is amplified by two performances of acutely familiar humanity by Jessie Buckley and Riz Ahmed, ideally matched as love-laboratory technicians whose growing mutual attraction defies the science.