Charlotte Doyle has spent several years of her life in England, as her father's job as an American business agent brought the family there when Charlotte was six. She is now thirteen years old, and her father has been recalled to America and given a promotion.

However, when Charlotte's parents and brother go back to Rhode Island, Charlotte does not go with them. She is in the middle of a school term at the Barrington School for Better Girls, and her father does not want her to miss any schooling. She will finish the spring term and return to America in the summer when school is not in session.

Charlotte is supposed to travel with two families of her parents' acquaintances, and she is looking forward to spending time with the children during the ocean voyage. On the day of departure, however, Charlotte boards the Seahawk alone. The other families have canceled their passage, she is told, and Charlotte will travel by herself, crossing the sea with only the captain and sailors for company.

At this point, Charlotte wants nothing more than to return to school, but Mr. Grummage, her father's business associate, will not hear of such a thing. His job is to get Charlotte on the ship at Liverpool, and he will not deviate from his orders one bit. Charlotte is taken on board the Seahawk and left alone. Thus begins the most fantastic adventure of her life.