Carey Mulligan’s range is a thing of wonder. If you’ve already seen her as an avenging American in “Promising Young Woman,” watching her in “The Dig” may induce something like whiplash. Here she portrays, with unimpeachable credibility, Edith, an upper-class English widow and mother in the late 1930s who is fulfilling a dream too long deferred.

The dream is to dig up her backyard. It’s a big one, mind you, on her estate in Suffolk, dotted by what appear to be ancient burial mounds. To this end, Edith, whose youthful interest in archaeology was squelched on account of her sex, hires Basil Brown, a determined freelance archaeologist played with stoic mien and working-class-tinged accent, by Ralph Fiennes.

Once the work begins, it becomes clear that something big is underground — this movie by Simon Stone, and the novel upon which it’s based, is a fictionalized account of the discovery of the treasure-filled Sutton Hoo, one of the biggest archaeological finds of the 20th century.

Brown’s crew increases, taking in a dashing cousin of Edith’s (Johnny Flynn, bouncing back from the grievous “Stardust”) and a discontented married couple (Ben Chaplin and Lily James). Big Archaeology tries to horn its way in. Much drama ensues.

Weighty themes are considered here: the question of who “owns” history; the corrosive effects of class inequality; the potentially tragic intertwining of sexual repression and loneliness. To its credit, this consistently interesting and at times engrossing picture declines to strike any of its notes with a hammer. Trading on the great British art of understatement, it’s scrupulous, sober, and tasteful throughout.

The Dig
Rated PG-13 for themes and language. Running time: 1 hour 52 minutes. Watch on Netflix.