Alex Pettyfer, a former model whose previous movie roles have played on his hunky good looks (see “Magic Mike,” for example), goes for something quite different in his directorial debut “Back Roads,” a grim, slow-moving period melodrama about a dysfunctional small-town family plagued by dark secrets. Presented like a Greek tragedy transposed to 1990s Pennsylvania—you might compare it to two Anthony Perkins pictures loosely based on Euripides, “Desire Under the Elms” and “Phaedra”—the morose, brooding film is certainly a change of pace for Pettyfer, but only a sporadically effective one.

Pettyfer plays Harley Altmyer, a shell-shocked twenty-something forced to become a surrogate parent to his three younger sisters—Amber (Nicola Peltz), Misty (Chiara Aurelia) and Jody (Hala Finley)—after his mother Bonnie (Juliette Lewis) is convicted of killing their father. Working the night shift at the local foodstore, he meets Callie Mercer (Jennifer Morrison), a housewife trapped in an unfulfilling marriage, and before long they will be involved, with Harley growing increasingly possessive even as Callie worries about what exposure might do to her relationship with her husband (Tom Everett Scott) and children.

The affair ends in disaster—no surprise, as the script by Tawni O’Dell and Adrian Lyne, based on O’Dell’s novel, employs a framing device in which a disheveled Harley is being questioned by grim Sheriff Mansour (Robert Patrick) about why he killed his lover.

There are, nevertheless, grim revelations galore during the journey along the back road that leads to the isolated family home. Harley is benumbed by the situation he finds himself in: he reveals little to his state-appointed therapist Betty (June Carryl), whose sessions with him periodically interrupt the narrative, and he stumbles through his job somnolently, sometimes so unkempt that his boss tells him to go take a shower. His contempt for Bonnie is palpable: on the rare occasions when he visits her in prison, he either ignores her efforts to connect or quietly berates her.

At home he’s unable to maintain any control. Amber has become a hellion, a sexpot who throws her dalliances with seedy local guys in his face, and while Jody seems reasonably well-adjusted, there’s something not quite right with Misty’s obsession with the rifle her father taught her to hunt with. The family’s financial situation is also desperate: mortgage payments are coming due, and though an uncle (Robert Longstreet) stops by occasionally, though apparently only to observe the girls, and remark on how much work needs to be done on the house.

As the film proceeds—at a glacial pace, it should be noted—the truth about the Altmyer family’s past is gradually disclosed, and it definitely isn’t pretty. The most important element involves why the deceased husband and father was killed, and why Callie died as well.

The main figure in all this is Harley, whose depression over the ruin of his life is understandable. Pettyfer portrays him as a near-zombie, emotionally desiccated until he meets Callie—and then goes overboard. It’s an effective job of playing against type, even if his effort to hold in his natural energy occasionally comes across as very stressful. By contrast Lewis’ pitiful, sometimes frenzied turn as Bonnie is powerfully affecting—something that cannot be said of Peltz’s performance as Amber: her shrillness may be character-driven, but it’s also unvarying and wearisome. Morrison, by contrast brings some genuine nuance to Callie.

The remainder of the cast is fine if unremarkable, which might also be said of the technical contributions of cinematographer Jarin Blaschke and editor Kant Pan, who certainly seems in tune with the director’s inclination to deliberation.

One can sympathize with Pettyfer’s desire to stretch artistically, and with “Black Roads” he has succeeded in some measure. But however sincere his film’s desire to grapple with very real social issues might be, its tendency toward melodrama ultimately hobbles its impact.