From left: Bernice (LisaGay Hamilton), Suarez (Edward James Olmos) and Fontayne (Yolonda Ross) navigate the Chinatown section of Tijuana in search of Bernice’s missing son in a scene from John Sayles’s detective story “Go for Sisters.” (John Castillo/Variance Films)

For the past several years, independent film icon John Sayles has proven to be one of cinema’s most reliable, occasionally brilliant, regionalists, creating rich, complex portraits of linguistic and geographic subcultures from Texas (“Lone Star”) to Alaska (“Limbo”), Florida (“Sunshine State”) and beyond.

In “Go For Sisters,” Sayles takes viewers to southern California and Tijuana, where a daily trade in people desperate to cross the border for a better life keeps all manner of mules, brokers, corrupt federales and common thieves in brisk business. It’s that murderous demimonde that a Los Angeles probation officer named Bernice Stokes (LisaGay Hamilton) finds herself navigating when her life as a straight arrow takes a wholly unexpected turn.

Her compatriots on the unlikely journey are an old friend named Fontayne (Yolonda Ross) and an ex-cop named Suarez (Edward James Olmos), both of whom have more experience with the rougher precincts they end up visiting, but neither of whom possess Bernice’s particular brand of grit.

For all his considerable gifts as a writer, Sayles has succumbed to schematic plotting, too-obvious contrivances in recent films, and unfortunately “Go For Sisters” is no exception. After an intriguingly evocative opening sequence during which we observe Bernice on the job, where she bumps into Fontayne after a years-long estrangement, her journey takes on the tidy, over-determined contours of a sub-par detective story.

But “Go For Sisters” is worth the time if only to witness the terrific chemistry between Hamilton and Ross, the latter of whom delivers a break-through performance as a woman of uncommon, almost regal, composure, even as she struggles to stay on the righteous path.

As their characters revisit their shared past and reflect on how theirs lives diverged, “Go For Sisters” possesses the easy, spontaneous naturalism for which Sayles is rightfully worshipped and adored in indie circles. (The movie’s title refers to Fontayne’s recollection that they could have been mistaken for siblings when they were younger.) Granted, once Olmos joins the proceedings, “Go For Sisters” begins to resemble a banal prime-time cop show. Then again, considering his mesmerizing co-stars, that’s a series I’d happily watch any time.

★ ★

Unrated. At West End Cinema. Contains profanity, brief violence and adult themes. 123 minutes.