In dire financial straits, the physician-researcher, Dr. Malcolm Sayres (Robin Williams), accepts a clinical job for which he is decidedly unsuited: staff physician in a chronic-care hospital. His charges include the severely damaged, rigid, and inarticulate victims of an epidemic of encephalitis lethargica. Sayres makes a connection between their symptoms and Parkinson’s disease. With the hard-won blessing of his skeptical supervisor, he conducts a therapeutic trial using the new anti-Parkinson drug, L-Dopa.

The first patient to "awaken" is Leonard Lowe (Robert De Niro) who, despite being "away" for many years, proves to be a natural leader, with a philosophical mind of his own. Other patients soon display marked improvement and their stories are told in an aura of fund-raising celebration marked by happy excursions.

Gradually, however, problems develop: patients have trouble adapting to the radical changes in themselves and the world; Leonard grows angry with the imperfection of his rehabilitation; the horrifying side effects of L-Dopa appear; and Leonard’s mother (Ruth Nelson), initially happy for her son’s recovery, is later alienated by the concomitant arousal of his individuality, sexuality, and independence. The film ends with "closure of the therapeutic window" and marked regression in some patients, but not before they have awakened clinical commitment and a new ability to express feelings in their shy doctor.