Plain City is a village in Madison and Union counties in the U.S. state of Ohio, along Big Darby Creek. The population was 4,065 at the 2020 census.

Madison County is a county located in the central portion of the U.S. state of Ohio. As of the 2020 census, the population was 43,824. Its county seat is London. The county is named for James Madison, President of the United States and was established on March 1, 1810.