In Mostly Harmless, there is not really a central antagonist in both Arthur’s and Trillian’s stories, but however, there is somewhat of an antagonist in Ford’s story. The Guide’s offices have been bought out by InfiniDim Enterprises, who have changed the format of the Guide dramatically. They are the central cause of alarm in the final chapters of the book. However, the issue isn’t fully resolved. The new Guide is actually sentient, and also drives Random to do certain questionable actions later on. So in short, the new Guide must be stopped, as stated to sound like a chiché from an old action movie.
Mostly Harmless 