This is a Cinderella story with a small but important difference. The difference: It is not a 'rags-to-riches-story.' Agnes is a poor girl who suffers separation from the family and goes through much sufferings at the place of work. She works as a governess. She holds on believing only in the Providence (after all Agnes is the daughter of a vicar). She prays more when she suffers more. And when she loses almost the hope, God apparently hears her prayers and she is united with her lover. The lover is not a rich man of high rank. He is a simple curate/vicar who is God fearing and a man of modest means. 
 Having read this summary of the story, one might think it is a simple story. And you are not wrong. It is a simple story. But it can be very uplifting. Or at least put you in good humour. It is a piece of literature that pleases the reader at the end. 
 However, a real look into the life of Anne Bronte will help the reader see beyond the story. For it is partly (Mostly, I suppose) autobiographical. Anne Bronte worked as a governess. The 'work' of the governess as mentioned in the book looked really horrifying. It was surprising that she kept her mind intact (I sincerely believe that she had lost it once when she killed the poor little birds with a big flat stone). Thus it is terrifying to note that the lunatics admitted in the asylums in those days had many number of governesses. Anne Bronte did a great deal to reveal to the world what was happening inside of the 'home school rooms'. I think that was one of her great contributions. 
 I sincerely wish that Anne Bronte had a happy ending like Agnes Grey had. Anne died young having not expressed her love. 
 Here is a passage from the book: "I have lived three-and-twenty years, and I have suffered much, and tasted little pleasure yet: is it likely my life all through will be so clouded? Is it not possible that God may hear my prayers, disperse these gloomy shadows, and grant me some beams of heaven's sunshine yet? Will He entirely deny to me those blessings which are so freely given to others, who neither ask them nor acknowledge them when received? May I not still hope and trust?" 
 Anne was twenty nine when she died having received no consolation that Agnes had received in the novel. My question to God: Why oh Lord, why?