“Fingernails,” for all its modern polish, has a dusty “Twilight Zone” premise. Picture, if you will, a machine no bigger than your average microwave. The wall-mounted rectangular box has the power to prove love between two partners yearning for certainty. Insert two Petrie dishes, each with a just-plucked nail as its sample, press a button, and, perhaps after a little smoke, the machine’s screen will reveal whether these two lovebirds are a 100 percent match, 50 percent, or nil.
We will put a pin in the question of whether romantic love can be quantified or if it’s a powerful emotion that can ebb and flow, and focus on the central character, a redhead with a wretched haircut, Anna (Jessie Buckley).