He agreed with a group of citizens who wanted to challenge unjust laws. As a test, Plessy violated the 1890 Louisiana Separate Car law. That means he agreed to break the law on purpose.
The Separate Car law said that white citizens and black citizens had to ride in separate railroad cars. Plessy had one African great grandmother. All the rest of his family was white. He looked white. When he boarded the "whites only" railroad car and handed his ticket to the conductor, Plessy had to tell the conductor that he was one eighth black. When he refused to move to the "blacks only" car, the conductor had him arrested. Plessy had to pay a $500 bond to get out of jail.