The state of Tennessee passed the Butler Act in 1922, making it a crime to teach evolution. Bertram Cates (Dick York, in the role that represents the real life John Scopes) is put on trial for teaching evolution to his high school students. The prosecution brings in Matthew Harrison Brady (in real life William Jennings Bryan), famous preacher and 3 time presidential candidate. The Baltimore Sun newspaper sends in their star writer E K Hornbeck (Gene Kelly, in real life H L Mencken). The sun pays for the defense to hire famous lawyer Henry Drummond (Spencer Tracy, in real life Clarence Darrow).
The trial is a farce. Prosecuting attorney Brady leads after hours prayer revivals for the spectators and jury. The judge refuses to allow Drummond to bring in any witnesses. Brady continually interrupts the proceedings to spout off little homilies. The judge gives in to anything the prosecution wants, and never upholds any of the defense motions. Many of William Jennings Bryan's real life quotes are included in the movie, such as, "It is better to trust in the Rock of Ages, than to know the age of rocks".
Cates is found guilty, and sentenced to a $100 fine with no jail time.
Note: In real life, Scopes was found guilty and fined $100, but the verdict was overturned by the Tennessee Supreme Court.
Thanks Joseph C!