Katy, a red crawler tractor, belonged to the Highway Department of the City of Geoppolis. She was not used in winter, however, until a huge snow storm snowed in the whole city. Then Katy plowed out everyone.
When Mike Mulligan and his steam shovel, Mary Ann, lose their jobs to the gasoline, electric, and diesel motor shovels, they go to a little country town where they find that one new job leads to another.
Maybelle loves to carry people up and down the hilly streets of San Francisco, until the City Fathers decide that she should be taken out of service in the name of progress.