In Paris, a middle-aged man (Bruno Todeschini) contracts a mysterious blood disease. He is in and out of the hospital, living under a vague yet looming death sentence. The man, a bit of an intolerant pill, is heterosexual; his younger brother (Eric Caravaca), who becomes his caretaker, is gay. Those facts of orientation are far from incidental to the disarming drama of Patrice Chèreau's Son Frère. Though the disease in question is not AIDS, it is very much meant to feel like AIDS. Chéreau's radical premise is that the illness still carries hints of stigmatization that can be undone by allowing us to experience it, metaphorically, as a ''heterosexual disease.'' Son Frère is hushed, clinical, grimly paced, and moving.