Ah, the tried-and-true "...from Hell" thriller subgenre, how it is impossible to quit even when the movies themselves lean toward the exceedingly formulaic. The latest entry, "Unforgettable," is the confidently envisioned directorial debut of prolific producer Denise Di Novi, and while it hits most of the usual narrative beats one expects, the third act effectively diverges from the strictly predictable. In an instant, much of what has come before and where everything has led reveals itself to be a little more psychologically complex than has initially met the eye in Christina Hodson (2016's "Shut In") and David Johnson's (2016's "The Conjuring 2") screenplay. A pair of ace performances from Rosario Dawson (2013's "Trance") and Katherine Heigl (2013's "The Big Wedding") don't hurt. Even if the film doesn't wholly live up to its lofty title, "Unforgettable" offers enough beneath the surface to lift it above the mediocre norm.

In one corner is Julia Banks (Rosario Dawson), a successful editor for an online storytelling site, who bids adieu to her life in San Francisco for a new one with fiancé David Connover (Geoff Stults) in his small Southern California hometown. In the other corner is David's Type-A-to-the-max ex-wife Tessa (Katherine Heigl), who cannot bear to see him or their young daughter Lily (Isabella Kai Rice) with a new beau and mother figure. Bred by her own domineering mom Helen (Cheryl Ladd) to present a constant external facade of perfection, Tessa feels wronged by a reality which betrays this illusion.

Anyone who has seen a fondly labeled "...from Hell" thriller of the "Unforgettable" sort will know, for the most part, what to expect next. Julia tries to be friendly with Tessa, who is clearly having a tough time letting go of her betrothed existence of domesticity, but their relationship becomes increasingly contentious as Tessa invades her privacy and uses her new knowledge to play mind games with her. There is a stolen cell phone and a missing irreplaceable engagement ring. Social media identity theft. Damning false accusations. A discovery of a secret abusive relationship in Julia's past that Tessa uses to her devious benefit. Then things grow violent.

The story's trajectory is routine until it finally is not, and the getting-there, while commonplace, is diverting in a lazy Sunday kind of way. In less assured hands, the picture could have easily come off as sexist, pitting one female who has traded exciting city life to be with a man against another female who has no career and bases her entire self-worth on rekindling her failed marriage. Director Denise Di Novi is adept enough, however, to not let this happen, coloring both characters with more than simply broad strokes. Julia retains her profession (she begins working remotely), and tries to empathize with Tessa's struggles rather than write her off. It soon becomes apparent that Tessa is suffering from mental illness, her psychopathy the likely perfect storm of an untreated chemical imbalance and the oppressiveness with which she was parented well into adulthood. Every time she flirts with turning into a one-note villainess, the film throws in moments that make a point of understanding who she is and why her actions, as criminal as they become, are those of a person in desperate need of help.

In her first major film role in several years, Katherine Heigl tears out of the gate with a ferociously understated performance. Her Tessa does despicable things, and by the end the viewer actively wants her to receive comeuppance, but she is neither a caricature nor a monster. Heigl successfully navigates this tricky balance, both believably unhinged and also poignant as she moves toward a fate as unexpectedly operatic as it is emotionally honest. Rosario Dawson may be cast in the less flashy role of put-upon protagonist Julia, but she is strong, relatable, and plays the part in a way that avoids victimization. Dawson has an innate talent for making everything she is asked to do believable rather than a necessity of the script, and this comes in handy here during a number of frustrating instances where coming clean and speaking the truth could solve most of her problems.

"Unforgettable" has no delusions of grandeur over the movie it is, and what it wants to achieve. This is a commercial crowd-pleaser, a well-acted, slickly shot B-movie providing ample thrills and comfort in its familiarity. Di Novi uses these established conventions as her playground, revealing a sharp eye and a care for her two female leads. Every time hope begins to fade for something deeper, she raises the bar on her treatment of the material. The climax, building to a series of crucial discoveries and confrontations, is rousing and suspenseful before finding sobering new layers in Tessa's resolve. Under different circumstances, Tessa might have been able to move on, to find a new purpose in her life, and to even remain on friendly terms with Julia. Their chemistry can be glimpsed in a scene where they share margaritas at a Mexican restaurant, but Tessa simply cannot let her pride or her jealousy go long enough to realize the woman sitting across from her could be an invaluable ally rather than an enemy. "Unforgettable" shrewdly sees the tragedy in this missed opportunity gone woefully sour. In her final moments, so does Tessa.