McGregor’s latest is more conventional in structure and approach. It opens with a literal cliffhanger. During a sudden Antarctic squall, three members of a British GPS mapping expedition become separated. One of them, the experienced guide and technician, Robert “Doc” Wright, slips and finds himself suspended precariously above a sheer drop. “The cliff wasn’t high but he wouldn’t survive a fall.” The others, Thomas Myers and Luke Adebayo, wind up, respectively, stranded on an ice floe and hunkered down beside a snowmobile. It’s a dire and dramatic opening scene, narrated with aplomb.