Ralph Fiennes arrives in a cloth cap at the door of a country house to begin The Dig. “Morn’,” he says to the butler in rustic Suffolk tones. He is here to see the owner. The correct response seems obvious: “It’s Ralph Fiennes, ma’am. He’s doing an accent.” But no, the butler plays along, introducing him instead as Basil Brown. A local archaeologist, Brown is drawn from real life, as is the owner of the house, Edith Pretty, a fragile widow played by Carey Mulligan. So too the story, or the gist of it anyway. The year is 1939, the property the now famous Sutton Hoo. A dark future takes shape above: RAF planes drilling on the brink of war. On the ground, Pretty has asked Brown to investigate her estate. She believes antiquities may lie beneath. “Speak dun’t?” he says. “The past.” It will not be the last time a major theme is stuffed into his mouth.

What a strange, frequently hackneyed, occasionally marvellous film this is. Sometimes The Dig is thrillingly subtle, sometimes as subtle as a spade to the head. At the most basic level, it tells the true story of Brown — formally untrained, an excavational savant — and the Anglo-Saxon discovery he makes on Pretty’s land. From the start, parody threatens, a national treasure hunt of pained polite silences, rain and jumpers. Yet for a time, the film feels wildly original — as if no one involved had ever before seen an English period drama. Much of that is down to the leads, their performances so vivid you feel them in the room with you. But director Simon Stone brings fresh eyes too. Flat Suffolk is shot with such widescreen grandeur it takes a while to realise how much is being borrowed from Terrence Malick. Who cares, you conclude. Forget the Texas Panhandle — gaze with awe at Woodbridge and Snape.

One sequence is a knockout for the ages, an underground Brown caught mid-sentence as the earth collapses on top of him. What follows is astonishing — pure cinema on Netflix or anywhere else. Yet Fiennes is endlessly watchable even when not in mortal danger. Although the script goes big on Arcadian wisdom, Basil rises above cliché. His wife May (Monica Dolan) is sharper still. But his odd kinship with Pretty is the heart of the movie. For all that Mulligan is too young for the role — Pretty was in her fifties by the time of these events — the actress is excellent, her Edith brittle but enduring. Together they make a surprise package, a matching pair of one-offs.

Which makes what happens next even more of a pity. In movie parlance, a sudden veer after 40 minutes into a whole other story is called doing a Psycho. So proceeds The Dig, lurching from two-hander to ensemble piece. Too many characters for a shower scene at least. The cue is the takeover of Sutton Hoo by the British Museum, a crew of career archaeologists descending. From here, the film is dazzled by the bright young things on site, notably the demure Peggy Preston (Lily James), newly wed to jolly academic Stuart Piggott (Ben Chaplin). The film shifts focus to unearthing secret passions, many involving Pretty’s bloody nice chap of a cousin, Lomax (Johnny Flynn). Tremulous longing abounds. Clichés? You’re needed on set after all. Basil and Edith look on, reduced to supporting parts. The lesson may be that audiences have limited time for middle-aged platonic friendships. Or perhaps just that producers do.

★★★☆☆

On Netflix from January 29