Not since John Sayles’s “Lone Star” has a film led up to so unexpected, yet in retrospect utterly inevitable a twist as Denis Villeneuve’s remarkable “Incendies.” The film is at once a stirring familial drama, a potent commentary on modern political strife and—more than a sideline—a cracking good mystery.

It begins with Montreal twins Jeanne (Melissa Desormeaux-Poulin) and Simon (Maxim Gaudette) hearing the contents of their late mother Nawal’s will from her employer, the family notary Jean (Remy Girard). He hands them sealed envelopes, one to be delivered by Jeanne to their father and the other by Simon to their brother. Since they assumed their father was long dead and were unaware they had a brother, the instructions shock them, especially because the will also stipulates that their mother is not to be properly buried until her children have completed their mission.

Simon reacts with rage and initially refuses any part in the matter, but Jeanne immediately heads off to Nawal’s homeland of Fuad, a fictional Arab country modeled after Lebanon, where she gradually uncovers the truth about her mother’s life before she emigrated to Canada. It’s a turbulent story, starting with a doomed love affair and continuing through a disastrous pregnancy, a conflict with Christian paramilitaries, a long stint in prison, torture and rape. In time Simon, along with Jean, arrives to finish off the search, which ultimately takes the trio back to Montreal.

The film intercuts scenes of the contemporary search with flashbacks to the story of Nawal (Lubna Azabal) from her teens to the present, ultimately revealing her tragic past and its ramifications in the present. It would be unfair to reveal any further details, since much of the excitement of “Incendies” lies in discovering the answers to the film’s many mysteries along with Jeanne and Simon. Suffice it to say that the trail is a convoluted one, involving family honor, religious partisans, revenge, and exceptional cruelty and kindness—as well as coincidences that might make the entire narrative seem absurd if they weren’t cannily handled.

Happily here they are. Villeneuve has adapted Wajdi Mouawad’s play for the screen very effectively, so that one isn’t at all aware of its stage roots. And his direction skillfully combines gritty realism with a heightened sense of drama for maximum effect. He also constructs some extraordinarily tense set-pieces, one on a bus and another that moves from a refugee camp to the secure enclave of a militia leader. And throughout Andre Turpin’s cinematography (along with Andre-Line Beauparlant’s production design) adds to the sense of authenticity and Monique Dartonne’s editing to that of urgency.

As do the performances. Azabal anchors the film as the long-suffering Nawal, never making a cheap play for audience sympathy and underplaying instead. Desormeaux-Poulin and Gaudette are also excellent as the twins, while Girard adds a welcome touch of easygoing levity to the notary. The remaining cast has clearly been carefully chosen and contributes finely-drawn support.

“Incendies” is a strange combination of political thriller reminiscent of Costa-Gavras with a women’s picture from the 1940s. One would hardly expect such an odd amalgam to work, but in Villeneuve’s hands it does, and beautifully. In the end you’ll be both surprised and satisfied.