Addie B. and her cousins, Jack, Beanie, and Bodie are mourning the loss of their grandmother Winnie. While visiting their grandfather's beach house in Maine, Addie receives a mysterious treasure box once owned by her grandmother. In it is a moonstone ring and a mysterious poem. The poem leads Addie and her cousins to nearby Moonstone Cave where together they enter a secret and magical garden and embark on an adventure of a lifetime. Along the way, they meet wonderful friends like Gemma, who reminds them all of a younger version of their grandmother; Jumani, a large white wolf; and Jadira, a strange and smelly creature whom they grow to love. During the adventure, they learn the most important lesson of all - they must work together if they want to escape the dangers ahead of them and find their way back home.