This was Audrey Hepburn's last starring role, and the movie that drove director Peter Bogdanovich into bankruptcy (according to IMDB).
To quote from the NY Times review from its initial release "You cannot believe that this is the work of the same man who made ''The Last Picture Show,'' ''Paper Moon,'' ''Daisy Miller,'' and ''What's Up, Doc'' ."

They All Laughed

John Russo (Ben Gazzara), Charles Rutledge (John Ritter) and Arthur Brodsky are private investigators working for the same agency.

The husband of Angela Niotes (Audrey Hepburn) and the husband of Dolores Martin (Dorothy Stratten) have each hired their agency to follow their wives. However, John falls in love with Angela, and Charles falls in love with Dolores.

They each blow their cover and become acquainted with the woman they are following. To complicate matters further, John's girlfriend Christy Miller realizes he has fallen for some one new, and goes after Charles herself. Dolores is being unfaithful with rich young South American playboy Jose.

Angela leaves Manhattan with her husband. Christy and Jose end up getting married. Dolores divorces her husband to go with Charles. Cabdriver Sam (Deborah Wilson) picks up John and wants to pick up with him now that Angela is gone.

Thanks Joseph C!