Loosely based on Louis Nowra's own experiences as a young theatre practitioner, Cosi follows protagonist Lewis, an unemployed actor who joins a government program to help rehabilitate mentally ill patients in a Sydney institution. Among the patients are psychopathic firebug Doug, pathologically shy Henry, lovesick Cherry, forlorn Ruth, recovering drug-addict Julie, and ever-demanding Roy. Hoping to realise his dream of staging a production of Mozart's Così Fan Tutte, Roy begins takes over the inexperienced Lewis's program, even though oblivious of the fact that none of the other patients can sing opera. As Lewis attempts to keep control of the inmates, his own life begins to spiral out of control. After being dumped by his girlfriend, Lewis finds himself drawn to Julie. He is also sacked by the hospital administrator. Having come so far with their production, however, he and the actors secretly rehearse at night. Errol, the head of nursing, tells his supervisor that the group is preparing a variety show to be performed for the health minister's forthcoming visit. On the night, they instead present their somewhat chaotic version of Cosi.