In the case of Nichols v. Universal Pictures Corp. (45 F.2d 119 (2d Cir. 1930)), Nichols had written a play about an Irish guy who marries a Jewish girl. Universal made a movie where an Irish guy married a Jewish girl. Although the stories and characters were very different, Nichols still sued for copyright infringement. The Court said that you can't copyright general themes or plots. There is no infringement unless the works are substantially similar.