You aren’t privy to the happier times for Miriam and Antoine Besson in “Custody,” Xavier Legrand’s searing drama about the separated couple battling for the guardianship of their son Julien. It’s hard to imagine such a moment ever existed as the two are introduced sitting largely expressionless before a judge with their lawyers hashing things out, Miriam’s detailing a history of physical and emotional abuse that led Julian to give a statement on her behalf while Antoine’s ignores what the rest of the family has to say to promote the testimony of co-workers who say he’s “an excellent team leader.” The disparity in the quality of who speaks up for who is striking, but as Legrand shows so subtly, laws don’t allow for nuance and when the judge rules for the Bessons to have joint custody of Julien, requiring the boy to spend weekends with his volatile father, it becomes clear in a situation where few things are that it’s a tragedy in the making.

Running a tight 90 minutes, there isn’t a wasted second in “Custody.” (In fact, Legrand’s 2013 Oscar-nominated short “Just Before Losing Everything,” detailing Miriam’s initial escape from Antoine, acts as prologue, though unnecessary to find his feature debut compelling.) The writer/director completely trusts Léa Drucker and Denis Ménochet, who play Miriam and Antoine, and the young Thomas Gioria, who plays Julien, to make the most of their screentime, often lingering in closeups as they all radiate a mix of frustration, anger and constant calculation to figure out how to get what they want out of a situation that’s untenable to all. A flashpoint is presented by the birthday party for the Besson’s older daughter Josephine (Mathilde Auneveux), who at 18 is thankfully just old enough to escape being subject to the the judge’s ruling, yet obviously not out of Antoine’s reach since she’s constantly fearful about a relationship she has with a fellow musician. Miriam has done her best to keep the details of Josephine’s birthday a secret from Antoine, but when Julien lobbies to go to the party on his father’s court-mandated time, “Custody” sets up for a breathless finale.

Although Legrand isn’t shy about portraying Antoine as the monster that he is, with Menochet’s sleepy-eyed giant demeanor likely never put to better use, not even as the dairy farmer who knows more than he lets on in his international breakthrough at the start of “Inglorious Basterds,” the writer/director is wise to recognize how his insidious behavior has infected the entire family, which has become accustomed to casually telling each other half-truths and lies as a form of protection, but often has the opposite effect of breeding more suspicion. While things grow more and more out of control for the Bessons, Legrand shows a master’s touch when it comes to what’s going on in the frame, steadily ratcheting up the tension and finding clever ways through the film’s sound design and pacing to illuminate the all-consuming horror of living in fear from a chronic abuser inside the contours of taut, electrifying thriller. As difficult as it is for Bessons to shake off the past to even start thinking of the future, “Custody” replicates the feeling so precisely that you won’t be able to stop thinking about it any time soon after seeing it.

“Custody” does not yet have U.S. distribution. It will next play at the San Sebastian Film Festival on September 25th and 26th and the London Film Festival on October 11th and 14th.