The title of Ana Piterbarg's moody not-quite-a-thriller Everybody Has a Plan doesn't apply to its protagonist, Agustín (Viggo Mortensen). He has an acute, unmooring malaise that has left him almost blank. A pediatrician in Buenos Aires, he drops out of his life and holes up in his study, growing a beard and growing strange.

Agustín, though, gets a second act built on an ugly deed. He assumes the identity of his twin, Pedro, and enters Pedro's world, the Tigre Delta, in search of he knows not what.

Ms. Piterbarg, who wrote and directed, makes the delta - a bayoulike landscape of mist-shrouded waters and far-apart dwellings - an eerie place out of time. With its lonely expanses, outlaw inhabitants (Pedro has been involved in various misdeeds) and bursts of violence, it could be the setting for an Argentine western. That's not a bad way to think about Everybody Has a Plan, in which the fight between good and evil plays out not at high noon but in Agustín.

Mr. Mortensen, who grew up in Venezuela and Argentina and speaks a fine, mumbly Spanish, does an excellent, unflashy job of making the two brothers distinct. The dead-eyed Pedro has an extra layer of scruff that the sad-eyed Agustín can't approach, even with his beard and his dingy sweaters and flannels.

And Mr. Mortensen keeps you watching, even when the movie's storytelling underwhelms. But Everybody Has a Plan is less about story than about texture and atmosphere. They stay with you, as does the haunted visage of Agustín, drifting on the delta waters.