Rosie the little mouse is treated like a princess in her house. One day she learns that she is going to be a big sister. She imagines all the fun she will have with a baby brother. When she learns that the baby is a girl, she experiences a wide range of emotions. Rosie's parents are preoccupied with the joy of having a new baby. Worried that she will be replaced as the 'Princess' in her house, Rosie sulks and refuses to acknowledge her baby sister, Blossom.On Blossom's first birthday, the town mice gather to celebrate, and Rosie's heart is broken when her father gives Blossom a tiny Cheddar Cheese Crown. When Blossom rejects the crown, the town is stunned, but then they hear Blossom say her very first word, which was her sister's name. Realizing that Blossom values Rosie more that the crown, Rosie's heart softens, and she falls in love with her little sister, "Ear-tips to toes."