After the death of their father, Maria and her younger brother Peter and goveness Miss Heliotrope travel towards Moonacre Manor to live with their Uncle Benjamin Merryweather. As they enter the sealed valley Maria finds her dreams coming to life: a little white horse, a huge dog and the boy she dreamt about so often. They find that the peaceful valley is in a family feud between the Blackhearts and Merryweather's dating back generations.
The townspeople of the Blackheart's land find that another of their wells have been infected with salt. Maria finds out the story of the two feuding families.
Maria seeks advice from Loveday, while Simon again confronts Benjamain about restoring their water supply. While out alone Maria is captured by Simon and he locks her up as her prioner in order to get the water supplied much to the displeasure of his mother who doesn't want the feud to rest.
Maria is let out of the room to share a meal with Lady Blackheart and Simon. Robin and Loveday devise a plan to rescue Maria, but in doing so she must revel her true identity to her son.
Maria, Robin and Loveday escape from the tunnels. After the argument with his mother, Robin is unable to forgive his mother. The Blackheart water supply is restored and the Moon Princess pearls found.
Maria takes the Moon Princess pearls to Father Francis and then decides to give them to Simon who should ultimately decide who they really belong to. Benjamin and Loveday are married.