George MacKay, center, in a scene from “1917,” directed by Sam Mendes. François Duhamel/Universal Pictures via AP

A soldier wakes up in a flowery meadow in the first few seconds of “1917,” Sam Mendes’ gripping action drama about World War I. To call this image the opening shot isn’t quite right, because “1917” contains essentially just one shot, a continuous stream of imagery that flows nearly uninterrupted for two hours. Though we know the film has technically been spliced together, the effect is undetectable (save for a brief cut to black). It’s an astonishing achievement from Mendes and his crew, notably editor Lee Smith and cinematographer Roger Deakins.

In fact, the filmmaking takes star billing over the story, written by Mendes with Krysty Wilson-Cairns. Like Christopher Nolan’s “Dunkirk,” Mendes’ movie wants to drop you into the middle of war to run, hide and fight along with your fellow soldiers. Both movies succeed admirably, and “1917” has an emotional core that “Dunkirk,” in its excitement, forgot to include. Still, this is a movie about sight and sound; heart and soul come second.

That sleeping soldier is Schofield (George MacKay), who has the bad luck to be napping near his buddy Blake (Dean-Charles Chapman). A superior taps Blake for a mission, Blake taps Schofield for a partner, and the two Lance Corporals end up racing across enemy territory to deliver an urgent message to Colonel Mackenzie (a brief but forceful Benedict Cumberbatch). His roughly 1,600 British soldiers are about to walk into a German trap, and Blake’s older brother is among them.

What follows is a journey through the horrors and thrills of war, some of them familiar (a slog through a sea of corpses), some of them eye-catchingly new (tank shells, standing on end, look like giant bullets that dwarf the two soldiers as they walk by). Events unfold in real time, taking us from the hellish center of a burning city to the freezing rapids of a river within seconds. It’s Schofield, a moper and complainer (as opposed to the plucky, can-do Blake), who will change the most over the course of the film.

“1917” feels like a close cousin to the “The Revenant,” another grueling adventure story, and “Birdman,” which also presented itself as a single shot (both were directed by Alejandro González Iñárritu). The film’s overall choreography and composition — one sequence includes a distant airplane that eventually lands inches from the camera — are dazzling. “1917” should thrill fans of audacious, arduous filmmaking.