It would be easy to go wrong with a melodrama like "My Sister's Keeper." Its general premise—that of a family fighting to save the life of a cancer-stricken child, and then coming to terms with letting her go—is neither fresh nor revelatory, and overly conventional storytelling could instantaneously transform it into a disease-of-the-week telepic. Worse still, soapy acting and a ceaseless string-laden score could have led to an insufferably mawkish and manipulative experience. Fortunately, the screenplay by Jeremy Leven (2004's "The Notebook") delves deeper than one might initially expect, and director Nick Cassavetes (2007's "Alpha Dog") takes all the necessary steps to ensure that this isn't some cheap, dime-store tearjerker. By choosing a more low-key, character-oriented approach and focusing on honest human conflict and relationships, Cassavetes mostly avoids pressing sentimentality. The cast members—every last one of them—are on equally strong ground. The results are tough, touchy, provocative, and exceptionally moving.

Anna Fitzgerald (Abigail Breslin) was conceived through genetic engineering for a very specific purpose: to be able to donate her blood, bone marrow and spare organs to older sister Kate (Sofia Vassilieva), a sufferer of acute promyelocytic leukemia. Now 11, Anna is confronted with potentially giving up one of her kidneys to a gravely ill 15-year-old Kate—a daunting procedure that will compromise her own health with no guarantees that it will help the recipient. Though the family of five is close—besides Anna and Kate, the Fitzgeralds' include lawyer-turned-stay-at-home mom Sara (Cameron Diaz), firefighter dad Brian (Jason Patric), and 16-year-old brother Jesse (Evan Ellingson)—they have also sacrificed a lot, their lives revolving around helping and caring for Kate. When Anna seeks the assistance of hotshot lawyer Campbell Alexander (Alec Baldwin) to legally acquire from her parents the rights to her own body, a steadfast Sara feels betrayed by the news and refuses to understand Anna's position. As Brian tries to mediate the situation and a lost Jesse drifts further into the background, mother and daughter find themselves facing off in court. Meanwhile, Kate quietly makes peace with her own inevitable death, and wishes the rest of her family could, too.

Based on the best-selling novel by Jodi Picoult, "My Sister's Keeper" alternates between the differing points-of-view of all its central characters, each one narrating their own opening segment. In doing this, director Nick Cassavetes does not lose sight of the picture's familial unit and their own personal struggles as they deal with the situation life has dealt them. Furthermore, the film adopts an enlightening non-linear structure that moves back and forth through time, with flashbacks helping to inform the present-day material. Touching upon heavy-hitting subject matter and the moral quandary that Sara has found herself in as she ruthlessly tries to save the life of one daughter without wanting to alienate or lessen the importance of the other, the movie questions whether Sara is ethically out of bounds in the baggage she has placed on the shoulders of Anna. Should Anna be able to have a say in what is done to her own body, and is she being at all selfish in her refusal to physically sacrifice any more for Kate? At 11, is it fair for Anna to even have to deal with such a burden? Some of the answers to these dilemmas are a given, but others are not quite so black and white, and give credit to the story for taking the time to even ask them at all. Even if Sara takes an inordinately long amount of time to relate to where Anna is coming from, there is a reason for her to be suspicious about her daughter's forthright beliefs. A third-act revelation makes perfect sense without feeling like a gimmick.

Were the plot a simple chronological telling of the same plot, it would be miserably downbeat as the viewer watches the deterioration of Kate as she is physically and emotionally ravaged by her disease while also contending with the ultimate tragic end to her first love with a fellow cancer patient, Taylor (Thomas Dekker). By moving around through the course of several years, seeing the family in happier times and in their early hardships as it relates to where they end up, the film's impact holds more force without being a study in morbidity. Sara gives up her career to do everything she can for Kate. Her marriage wavers unevenly as Brian's work often overtakes his responsibilities to the rest of the family. Anna cares for Kate more like the way a big sister might care for her younger, more vulnerable sibling. Jesse, the oldest child, is okay with his parents focusing on Kate's welfare, but it has come at the expense of him feeling loved, appreciated, and, for that matter, noticed at all. And then there's Kate, who wishes to experience all that she can with the limited amount of time she has left. Her strength and spirit are an inspiration, and the fear that sometimes shines through is authentic to someone in her position.

Performances are first-rate. Playing the very first parental role of her career, Cameron Diaz (2008's "What Happens in Vegas") slides with ease into the part, forcefully making her characters' points without falling into animated histrionics as Sara's desperation to save Kate grows. In a lesser script, Kate might have been cast as the villain of the piece. Instead, she is someone who simply wants to protect Kate so much that it is difficult for her to see the bigger picture and how her actions affect the rest of the family. A late scene between herself and her eldest daughter in a hospital room is heartbreakingly handled by Diaz. As husband Brian, Jason Patric (2007's "In the Valley of Elah") brings a quieter intensity to his portrayal of a man who loves his family and has his own opinions on how things should be handled, but is selective in the battles he picks. Abigail Breslin (2008's "Nim's Island") continues to impress and mature in her acting skills as she herself gets older, and her role as Anna might be one of her most demanding, to date. She doesn't strike a false note.

As brother Jesse, Evan Ellingson (2006's "Letters from Iwo Jima"), like Patric, is a forceful presence even when he has little to say. As Jesse is set adrift, free to pretty much do as he pleases, Ellingson touchingly displays with no more than his movements and facial expressions the emotional toll he is experiencing that no one else sees. Joan Cusack (2009's "Confessions of a Shopaholic") is exceptional as Judge Joan De Salvo, who takes on the case. Joan connects with Anna's plight because she sees her own young daughter, recently killed by a drunk driver, within her. Cusack's reaction is nothing short of devastating in a scene where Anna asks the judge how she felt when her daughter died. And last, but certainly not least, Sofia Vassilieva (TV's "Medium"), as Kate, is a standout in her first major feature film. This is a breakthrough turn that should garner her quite a lot of opportunities, and her commitment to the role (she agreed to really shave her head) and lack of vanity (the make-up department has done a startlingly realistic job) are both impressive and admirable. Vassilieva's depiction of a young girl who knows her life will be a fleeting one, and accepts her fate, has the ability to break your heart even as it comforts.

The concluding passages of "My Sister's Keeper" are sobering and uncompromising, but not without hope, and director Nick Cassavetes does a nice job of balancing the film's tricky tone. Soundtrack choices throughout range from the exquisitely placed (Regina Spektor's romantic "Better" and Greg Laswell's ruminative "Girls Just Want to Have Fun") to the cutesy and too on-the-nose (Don Ho's "Tiny Bubbles" as the family sits in the yard and, yes, blows bubbles), and one could do without a recurring reference to Anna, and later Kate, being described as "a little piece of blue sky." These criticisms, however, are a minor interference in a motion picture of notable sensitivity and beauty, hamstrung emotions kept to a minimum in exchange for the truth found in the story and characters. As a respite against the dumb comedies and mindless action pics that have recently been littering the mainstream studio landscape, the thoughtful "My Sister's Keeper" has what it takes to become one of the sleeper successes of the year. Now it's up to audiences to show up.