Agnes Grey is the daughter of Mr. Grey, a minister of modest means, and Mrs. Grey, a woman who left her wealthy family and married purely out of love. Mr. Grey tries to increase the family's financial standing, but the merchant he entrusts his money to dies in a wreck, and the lost investment plunges the family into debt.
Agnes, her sister Mary, and their mother all try to keep expenses low and bring in extra money, but Agnes is frustrated that everyone treats her like a child. To prove herself and to earn money, she is determined to get a position as a governess. Eventually, she obtains a recommendation from a well-placed acquaintance, is offered a position, and secures her parents' permission. With some misgivings, she travels to Wellwood house to work for the Bloomfield family.
The Bloomfields are rich and much crueller than Agnes had expected. Mrs. Bloomfield spoils her children while Mr. Bloomfield constantly finds fault with Agnes's work. The children are unruly and Agnes is held accountable for them despite being given no real authority over them. Tom, the oldest Bloomfield child, is particularly abusive and even tortures small animals. In less than a year, Agnes is relieved of her position, since Mrs. Bloomfield thinks that her children are not learning quickly enough. Agnes returns home.
She then begs her mother to help her find a new situation. Agnes advertises and is given a position in an even wealthier family â the Murrays. The two boys, John and Charles, are both sent to school soon after her arrival, but the girls Rosalie and Matilda remain her charges. Matilda is a tomboy, prone to lying. Rosalie is a flirt. Both girls are selfish and sometimes unpleasant, and although Agnes's position is slightly better than it was at Wellwood house, she is frequently ignored or used in the girls' schemes.
Agnes begins to visit Nancy Brown, an old woman with poor eyesight who needs help reading the Bible; there Agnes meets the new parson, Mr. Edward Weston. The next day while on a walk Agnes is surprised by Mr. Weston, who picks some wild violets for her. Agnes later saves the flowers in her Bible. She learns that his mother has died not long ago. This new friendship is noticed by Rosalie Murray, who has now entered into society and is a favorite with nearly all suitors in the county.
Rosalie becomes engaged to Lord Ashby, a wealthy baronet from Ashby Park. She tells Agnes, but makes her promise to keep silent, as she is still going to flirt with other men before she is married. One day, she and Agnes go on a walk and meet Mr. Weston. Rosalie begins to flirt with him, much to Agnes's chagrin.
Agnes receives a note from her sister Mary, who is now married to Mr. Richardson, a parson of a rectory near their home. Mary warns that their father is dying and begs Agnes to come. Agnes arrives too late to see her father alive. After his funeral, Agnes opens a small school with her mother, leaving behind the Murrays and Mr. Weston.
She receives a letter from Rosalie who is very unhappy in her marriage and asks Agnes to come for a visit. Agnes is shocked by the change in Rosalie from a merry girl to an unhappy young woman. Rosalie confides that she despises Lord Ashby (and her mother-in-law), and claims he only left London because he was jealous of all the gentlemen she was attracting. Agnes also hears that Mr. Weston has left the area, and she grieves, believing she will not be able to see him again.
Agnes leaves Ashby Park and returns home. The day after she arrives, she goes for a walk on the sea shore and encounters Mr. Weston, who had been looking for her since he moved to the nearby parsonage.
He is introduced to Agnes's mother, and they forge a bond. Agnes finds her attraction to him growing, and she accepts when he proposes marriage. In the end, Agnes is very happy having married Edward Weston, and they have three children together.

Question: Why did Rosalie want Agnes to keep her engagement a secret?
Answer: Because she still wanted to flirt with other men
Alternative answer: She still wants to flirt with other men until she is married