In a steampunk world where humans and monsters co-exist, Suri is an orphaned girl who earns her keep with a traveling merchant camp. But she yearns to be a monster tamer. One day, Suri manages to tame a mysterious monster and, triumphant, she brangs to a young customer of her skill. Strangely, this spooks him, and he runs away, dropping a ball of golden twine. That night, he and his family return for the twine, and Suri realizes that they are catsiths, dangerous feline monsters that can disguise themselves as human!