It’s one of the oldest weepie plots in the book. A young woman, with a loving husband and young children, suddenly learns she’s terminally ill. She decides to keep her condition secret in order to spare her family and friends pain, while working to complete her own agenda–a carefully-compiled list that includes not only arranging happiness for the survivors as best she can, but also having a romantic extramarital fling (just to find out what it would feel like to be with a man other than her spuse). Hanging over everything, of course, is the dark cloud of her imminent demise–a gloomy inevitability nonetheless mitigated by her courage and concern for others. Pass the tissues, please.

What rescues “My Life Without Me” from the maudlin banality of a typical TV disease-of-the-week movie are several factors. One is the austere approach of wrier-director Isabel Coixet, who resists the inclination to give in to the bathos such a story naturally invites; the style of the picture is spare and almost ascetic. Another is the generally excellent acting, especially by Sarah Polley as Ann, the doomed wife. And a third is the naturalness of the dynamic between Ann and her husband Don (Scott Speedman), as well as their two daughters Penny (Jessica Amlee) and Patsy (Kanya Jo Kennedy). This quartet feels like a real family.

Unfortunately, the virtues aren’t quite enough to outweigh the picture’s failings. The most notable of these are some serious stumbles in Coixet’s writing. Alongside the strikingly refined, well-judged moments, such as a beautifully constructed and played scene in which a curiously reticent doctor (Julian Richards) informs Ann of her illness, are other episodes that come across as heavy-handed and stagy. Everything involving Ann’s mother (Deborah Harry) seems forced–particularly a long soliloquy in which she discourses melodramatically on her life and broken dreams. The same is true of the material dealing with Laurie (Amanda Plummer), Ann’s friend at her janitorial job, whose salient characteristic is an obsession with her weight and diet that’s intended to be comic relief but is instead drearily repetitive. Finally, there’s Ann’s unlikely romance with a rumpled but handsome engineer named Lee (Mark Ruffalo), whose eye she catches during a chance encounter in a laundromat. It’s not merely that their meeting is forced–wouldn’t Ann’s mother have a washing machine in the house behind which the young couple’s trailer resides?–but that it seems entirely too coincidental. (How many times has this sort of thing happened to Ann before? And isn’t it terribly convenient that it happens just now?) Setting aside those questions, though, the hushed affair itself is never really convincing, remaining more a plot device than a true act of passion despite the efforts of Polley and Ruffalo to put it across. Lesser plot elements–one featuring a braid-obsessed hairdresser, another Ann’s brief meeting with her incarcerated father (Alfred Molina) work slightly better, but only just.

Still, the radiance of Polley’s performance, which is marvelously judged and quietly affecting throughout, along with her fine interaction with the nicely laid-back Speedman and the two ebullient young girls, gives Ann’s familiar story greater emotional weight than one might expect. “My Life Without Me” doesn’t quite transcend the essential staleness of its premise, but thanks to her it comes very close to doing so.