Tony Rome (Frank Sinatra) is a private investigator who lives in a houseboat in Miami Beach. He finds a young woman Diana Kosterman (Sue Lyon), robbed in a seedy hotel. Her father Rudolph Kosterman (Simon Oakland) hires him to find out about the incident. Diana has also lost her diamond pin in the incident and she and her stepmother Rita Neilson Kosterman (Gena Rowlands) hire him to find the pin. When he does find the pin, it turns out to be a cheap copy and Tony believes that Rita is involved. He also thinks that a man called Nimmo (Robert 'Buzz' Henry) could be involved. He has to act fast as too many people are dying in the process.