Mirage (1965)

After a blackout in his office building, accountant David Stillwell (Gregory Peck) finds he can't remember the past two years of his life. Discovering that a rich humanitarian died after falling from the same building, and that there are shady gunmen out to get him, Stillwell must piece together the missing memories of his life. Enlisting the help of a rookie private eye (Walter Matthau) and a reluctant old flame (Diane Baker), Stillwell uncovers the mystery detail by unexpected detail.