“Let Him Go” is a slow-burn thriller that starts out as a melancholy look at loss and grief before evolving into a sea of shocking violence.

The two tones are so different but manage to work off each other thanks to writer/director Thomas Bezucha and a talented veteran cast headlined by Diane Lane and Kevin Costner.

Lane and Costner play Margaret and George Blackledge. George is a retired sheriff living on a Montana ranch with Margaret, their son James (Ryan Bruce), his wife Lorna (Kayli Carter) and their newborn grandson Jimmy.

When James dies in a horse-riding accident, it sends Margaret spiraling into grief. A few years later, Lorna remarries and disappears with her new husband and Jimmy to the Dakotas, prompting a concerned Margaret to convince George to pack up and go look for their grandson.

They discover that Lorna and Jimmy are in the grips of a dangerous family headed by a domineering matriarch (Lesley Manville) who is willing to do anything necessary to keep George and Margaret away from their grandson.

Bezucha’s work from Larry Watson’s novel feels a lot like “No Country For Old Men” from writer Cormac McCarthy. Bezucha’s previous work includes “The Family Stone” and “Monte Carlo” – two films that are a far cry from what is going on here but actually have underlying similarities.

Like “Monte Carlo,” this film makes the most of its beautiful setting with spectacular work from cinematographer Guy Godfree. The landscape provides a backdrop that gives “Let Him Go” and old Western feel in a way that is comforting yet at the same time foreboding to what is about to come.

And like “The Family Stone,” this film is rooted in family dynamics. Lane and Costner are so compelling in the lead roles that the audience is rooted in their quest to keep together what is left of their family.

Their determination really drives the first half of “Let Him Go,” but the film hits another level when Manville arrives.

Manville has always been an actress capable of any challenge, but I think this may be some of her best work yet – with a performance that is as terrifying as any in recent memory.

She adds an element to “Let Him Go” that I wasn’t expecting – an element that makes this one of the better thrillers of 2020.