A pony is running loose on the day of Lulu's sister's birthday. The story is about how they find the pony and how it changes the careful plans for the party. It's almost time for Rosie's birthday party. But there's a problem! A pony is running loose and Lulu and her dad, the local vet, have to rescue it. Can they find the pony? And what will happen if the naughty pony gets into more mischief at the party? It's lucky that Lulu has a plan!