Perhaps one of the most terrifying health prospects one might face is the waking nightmare of being trapped in one’s own body, mentally astute but helpless to act as the body eats itself. The utterly engrossing documentary I Am Breathing opens eight months after its thirtysomething British protagonist, Neil Platt, has been diagnosed with ALS/MND (Lou Gehrig’s disease), which quickly lays waste to his form while leaving him as intellectually sharp as ever. Married only a few years, and the father of an infant son, Platt and his story are inherently moving, and co-directors Emma Davie and Morag McKinnon wisely employ a light, unobtrusive touch as their camera captures the erosion of Platt’s health. Platt never gives in to self-pity, and is instead a funny, droll, philosophical subject. The flickers of sadness in his eyes as he prepares a memory chest for his son, Oscar, writes a fatherly letter to be read by his boy at a later date, and sometimes looks at his wife, Louise, are devastating. The everyday routines and sometimes exhausting healthcare rituals are fleshed out with home movie clips and photos (from Platt’s childhood and university days, and his courtship of and honeymoon with his wife.) The film’s emotional potency, in fact, lies in Platt’s relationship with Louise. They clearly adore one another, even as the strain of their lives occasionally leads them to be a little snappish in their exchanges—they are humans, not saints. And those few times that Louise is shown dabbing tears might well break your heart.