Cady Chaplin is an intensive-care nurse at Lenox Hill Hospital. She just turned thirty. Her closest friend at work is Karen Cunningham, who is twenty years older and made a mid-career turn from photography to nursing. When they met, five years ago, Chaplin and Cunningham hit it off immediately. They live in the same neighborhood—South Park Slope, in Brooklyn—and often take the subway together to the hospital, which is on East Seventy-seventh Street, in Manhattan.