When a director casts Frances McDormand in anything they’re already ahead of the game. The actor’s deeply knowing eyes and forbiddingly expressive face offer a kind of raw, captivating power that transfixes viewers immediately. However, most directors underestimate how difficult it can be to fulfill the promise that McDormand’s performances broadcast from the moment she walks on screen. This was the problem with a movie like “Three Billboards Outside Ebbing, Missouri,” which served up a feast of McDormand’s mesmerizing rage, but then had no idea where to take it or what to do with it.

“Nomadland” presents a similar problem. At first director Chloe Zhou makes the most of a restrained, searching performance from McDormand to illuminate a subculture of American wanderers and self-exiles. It’s fascinating reportage and plays into Zhou’s strengths as a sophisticated observational filmmaker. But things run aground when Zhou realizes that she’s shooting a road movie, and road movies need to lead somewhere, and the resulting plot threads and interplay with other characters are both far less interesting and far more predictable than the first half. By the end, the sentimentality (or even lack thereof) becomes tedious and pro-forma.

In many ways, “Nomadland” feels like a prototype for a new kind of Oscar-bait: a benignly apolitical investigation of a subculture populated by non-actors serving as a backdrop for a powerhouse acting performance from a genuine star. McDormand gives everything she can to make it work, but Zhou has no idea what to do with it.