MARY-CLAIRE KING is an American woman who did something extraordinary. After years of painstaking research, she discovered the BRCA1 gene, the inherited mutation that indicates an elevated risk for breast and ovarian cancer, proving that such cancers are passed down from generation to generation.

"Decoding Annie Parker" focuses on King's efforts, in part, but doesn't explore them nearly as fully as they deserve. Early on, this based-on-real-events drama sets its course on two narrative paths. One follows King (Helen Hunt) and her team during their nearly two-decade quest to find that cancer-related genetic link. The other follows Annie Parker (Samantha Morton), a Toronto wife and mother whose family is shattered brutally and repeatedly by the disease. As the movie progresses, the balance tips increasingly toward Annie's story, turning what could have been a double-sided portrait of two women's perseverance in pursuit of an unknowingly shared goal into a sometimes moving but mostly familiar look at a single, lifelong struggle with the c-word.

As directed by Steven Bernstein, a veteran director of photography who makes his feature-film debut here, "Decoding Annie Parker" frequently dangles on the precipice of falling into Lifetime Original Movie territory. What saves it, over and over again, is its excellent cast, anchored by Morton as Annie, a woman who marries, gets pregnant and eventually discovers that her aspiring rock-god husband (Aaron Paul) can't cope when chemotherapy creeps into their lives. There's both a gentleness and a charcoal-hard stubbornness in Morton's portrayal that makes Annie naturally empathetic yet above pity. She's clearly a fighter, and Morton inhabits her with understated dignity and an admirable willingness to put the ugliest aspects of illness on display.

Unfortunately, as strong as Morton and her castmates - including Hunt, Corey Stoll, Rashida Jones, Alice Eve and "West Wing" veterans Richard Schiff and Bradley Whitford - may be, there's a gnawing sense throughout "Decoding Annie Parker" that the deeper, more interesting narrative isn't being told. Annie's cancer story, and all the romantic relationship drama that comes with it, sucks up all the emotional juice, but surely King's groundbreaking research, which took 17 years to come to fruition, involved struggle, too.