September Affair is the story of David Lawrence (Joseph Cotten), a married businessman, and Marianne Stuart (Joan Fontaine), a classical pianist, a couple who meet randomly on a flight to New York to Rome. Falling instantly in love, David and Marianne deplane during an unscheduled stop in Naples to spend a couple of days together before going back to their ordinary lives. While in Naples the couple hear a news story reporting on the crash of their original flight, with all on board presumed dead. When a clerical mix up lists David and Marianne among those presumed dead, the new lovers callously decide to leave their unhappy marriages in the past; never informing their families they are alive. When David's wife Catherine (Jessica Tandy) arrives in Naples to investigate her husband's last days, David and Marianne's secret is finally revealed. Will their romance survive the interference of their past lives?