Early in To the Bone, writer-director Marti Noxon's harrowing yet utterly approachable drama about eating disorders, Ellen (Lily Collins) considers a plate of food her stepmother has optimistically plopped in front of her. She runs down the calorie count: 280 for the pork, 350 for the buttered noodles, 150 for the roll, and 75 for butter. Ellen and her stepsister share a joke about having "calorie Asperger's," but she's already thinking about how she's going to burn the meal off, and then some, through running around the neighborhood or doing push-ups furtively in her bed after dark. On the latter, a specialist immediately recognizes the telltale bruises on her abdomen. He knows all the tricks.

It comes as no surprise to learn that Noxon, a veteran TV writer making her feature debut, based To the Bone on her own experiences with anorexia, because the film's strongest scenes are flush with "tricks" and other specific insights that stem from experience. In the in-patient outpost where much of the film take place, every patient has patterns of behavior — some of which overlap, others of which are unique to them, and all of which Noxon recognizes as private rituals of self-destruction. And it's not just the hows she knows but the whys, too — those psychological blocks that make family dinner at the dining room table feel like a trip to the rack.

As To the Bone opens, Ellen has just been booted from her fourth in-patient facility and may not survive to see a fifth. In a last-ditch effort, her stepmother Susan (Carrie Preston) arranges an appointment with Dr. William Beckham (Keanu Reeves), an unconventional and highly sought-after innovator in the field. Beckham places Ellen in a house where she and others suffering from various eating disorders share intimacies and abide by a set of firm guidelines and points-based incentives. Once there, she establishes a close relationship with Luke (Alex Sharp), a dancer and fellow "rexie" who's made progress after six months in the house, but still clings tenuously to his recovery.

Fans of Noxon's work on Buffy the Vampire Slayer and the first season of UnREAL —the second, without Noxon, went catastrophically bad — will recognize the tart, slashing wit and emotional volatility of her voice in To the Bone.Her expertise with group dynamics pays off in a house where sarcasm and gallows humor form the default mode, only occasionally punctured by moments of harrowing crisis. That was the prevailing mood on Buffy, too, where jokes were the best shield against a relentless onslaught of supernatural terror. For much of the way, Noxon's instincts as an entertainer make a difficult subject more accessible without minimizing its seriousness.

She also casts the film to the hilt. Collins commits herself physically to the role while adeptly slinging Noxon's caustic one-liners, and Reeves, as Ellen's doctor, strikes the right note between genuine sympathy and a sobering sense of where she might be headed. But Noxon stocks the supporting roles with a murderer's row of character actors, like Parks & Recreation's Retta as the house's stern but generous watcher, Lily Taylor as Ellen's aggrieved hippie mother, and Preston, whose shrewd daftness on The Good Wife gets redirected to seismic effect here. Even seemingly clueless suburban housewives have layers.

The only major problem with To the Bone, outside of a general indifference to style, is that Noxon can't quite crack the ending. As with any addiction, coping with eating disorders is a lifetime process; even if Ellen makes a full recovery, it's not like she'll suddenly forget the caloric value of every single foodstuff she ingests. Noxon may not intend to imply otherwise, but the too-neat conclusion doesn't suit a character whose psyche is full of serrated edges, even in the most optimistic of times.

A film ends. The work doesn't.

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