"Mistress" is a witty and chaotic comedy that follows the story of Marvin Landisman (Martin Landau), a veteran Hollywood producer who has fallen on hard times. One day, he comes across an old screenplay written by one of his former assistants, Bobby Korfin (Robert Wuhl), and decides to resurrect it as a comeback project.
Bobby's script is a quirky and romantic tale about a love triangle between a struggling writer, his boss, and a beautiful woman. Marvin sees the potential in the story but feels it needs a bit of tweaking to appeal to a wider audience. He sets out to find the money to fund the film but quickly realizes that he's not the only one interested in the project.
As he meets with various financiers, Marvin discovers that each of them has their own agenda and wants certain changes made to the script. Eddie (Danny Aiello), a crude and boisterous businessman, wants his mistress, a flamboyant drag queen, to be included in the story. Phil (Robert De Niro), a ruthless gangster, wants his own girlfriend to play the lead. And Jack (Eli Wallach), a charming but aging actor, wants his mistress, a young and beautiful aspiring actress, to be given a prominent role.
As Bobby struggles to balance the demands of the backers with his own vision for the script, he finds himself making more and more changes until the story is unrecognizable. Along the way, he meets a lovely and sassy waitress, Rachel (Sheryl Lee Ralph), who helps him navigate the chaos and stay true to his creative voice.
In the end, Marvin manages to secure the funding for the movie, and it becomes a huge success. But as the cameras roll, Bobby realizes that the film has become a parody of his original work, and he's heartbroken. He confronts Marvin, who admits that he was wrong and promises to make it up to him. The two men hug, and Bobby finds solace in Rachel's arms as they watch the movie together.