Dennis Meechum (played by Brian Dennehy) is a veteran police officer and crime writer struggling to come up with a new book idea. One day, he is approached by a professional hit man named Cleve (played by James Woods) who wants to tell his story to Dennis, in exchange for being a central character in Dennis' next book.
Cleve reveals that he used to work for a powerful politician named David Madlock (played by Paul Shenar) who used Cleve's services to eliminate his enemies. Cleve wants to reveal the truth about Madlock's criminal activities to the public, but he needs Dennis' help to gather the evidence and verify the story.
Dennis initially dismisses Cleve as another criminal, but when he sees the truth behind Cleve's story, he teams up with Cleve to gather evidence and confront Madlock. However, Madlock is not willing to let his criminal empire crumble, and he sends his ruthless henchmen to stop Dennis and Cleve by any means necessary.
As Dennis and Cleve travel around the country to gather statements and evidence, they become targets of multiple assassination attempts, explosions, and shootings. Meanwhile, Dennis' publishers pressure him to finish the book, despite the dangers it poses to his life.
In the final showdown, Dennis and Cleve confront Madlock and his henchmen in a dramatic shootout, where they use their investigative skills and fighting prowess to come out on top. However, the story has taken a toll on Dennis' personal life, and he has to make the tough decision of whether to publish the book or not.
Overall, Best Seller is a gripping crime thriller that explores the dangerous world of organized crime and the lengths people would go to in order to uncover the truth. The movie features amazing performances by Brian Dennehy and James Woods, who breathe life into their characters and keep the audiences at the edge of their seats throughout the movie.