While spending the summer at Grandmary's home on Goose Lake, Samantha and the twins Agnes and Agatha decide to visit the island where Samantha's parents were drowned during a storm.
When Samantha's tenth birthday party is spoiled by the boy next door, Aunt Cornelia and her young twin sisters try to ease Samantha's disappointment by inviting her and Grandmary to visit them in New York City.
Samantha is determined to help nine-year-old Nellie, attending school for the first time, with her school work and learns a great deal herself about what it is like to be a poor child and work in a factory.