When Granny Rae dies and leaves Ariella her precious unicorn charm, Ariella invests the charm with magic powers. Days later a unicorn appears in the empty field at the bottom of the garden. A young, gangly unicorn, with over-sized hooves, a scruffy mane and eyes like puddles of purple ink. Can the unicorn `magically\' heal the hole in baby Boo\'s heart? Can he stop Ariella being bullied at her new school? Can he help her to fit in? Or is the unicorn there because he needs help himself? Ariella and the `lost\' unicorn must embark on a journey together that will change them both, before their worlds part and they are forced to say goodbye forever.