In the case of plessy v. Ferguson, what did homer plessy refuse to do

Homer Plessy refused to sit in a Jim Crow car, breaking a Louisiana law.

This 1896 U. S. Supreme Court case upheld the constitutionality of segregation under the "separate but equal" doctrine. It stemmed from an 1892 incident in which African-American train passenger Homer Plessy refused to sit in a Jim Crow car, breaking a Louisiana law.