Molly Gibson, whose mother has long ago died, is brought up by her father Dr. Gibson. At the age of 17, Molly visits and immediately befriends the Hamleys, her neighbors, especially she and the youngest son Roger Hamley, studying in Cambridge, become good friends. This is when Dr. Gibson tells his daughter of his upcoming marriage to Hyacinth Kirkpatrick, a widower who also has a daughter around Molly's age, Cynthia.

Before going to Africa, Roger proposes to Cynthia and is accepted. Molly faces some problems with her stepmother and her views.

It is revealed that Cynthia is also secretly engaged to Mr. Preston ever since she was 15 years old. When she tries to break off that engagement, Preston is reluctant to let her go or give her back her letters. After a visit to London, Cynthia also breaks off her engagement to Roger who is on an expedition in Africa. Osbourne's health decreases quite severely. Fearing his father's reproach, he still cannot tell him of his marriage to a French woman or his having a child.

Roger returns from Africa after Osbourne's death to find his sister-in-law and his nephew living at their home as well. Cynthia has accepted the marriage offer of Mr. Henderson, a lawyer from London, while Roger seems to have his heart set on another.