It sounds as if the authors of a Dr. Seuss-”Star Trek” mashup will have to boldly go to trial on copyright infringement—without their fair use defense.

Argued before the U.S. Court of Appeals for the Ninth Circuit Monday, Dr. Seuss Enterprises v. ComicMix was framed by some amici curiae as a prime opportunity to apply fair use principles to the increasingly popular art of mash-ups. But the arguments ended up turning more on traditional principles of fair use, and not in a way that favored the authors of “Oh, the Places You’ll Boldly Go,” a book that places “Star Trek” characters and props in Dr. Seuss settings.