Klute is a 1971 film which tells the story of a prostitute who assists a detective in solving a mystery. It stars Jane Fonda, Donald Sutherland, Charles Cioffi, Dorothy Tristan, Vivian Nathan, and Roy Scheider. The movie was written by Andy Lewis and Dave Lewis and directed by Alan J. Pakula.

Klute was the first installment of what would informally come to be known as Pakula’s ‘paranoia trilogy’. The other two films in the trilogy are The Parallax View (1974) and All The President’s Men (1976).

The film begins with the disappearance of Pennsylvania executive Tom Gruneman. The police reveal that an obscene letter was found in Gruneman’s office. It was addressed to a prostitute in New York City named Bree Daniels (Jane Fonda), who had received several similar letters from Gruneman.

Much to the surprise of the police, Peter Cable (Charles Cioffi), an executive at Gruneman’s company, hires family friend John Klute (Donald Sutherland) to investigate the disappearance.

Summary via Wikipedia.