Casey is only fifteen years old when her stepfather tries to marry her to a perverted and brutal man twice her age. However, minutes before the wedding, Casey flees into the jungle where she is rescued from her pursuers by a mysterious woman who sets the terrified young girl on a path to becoming a pirate - a path full of adventure, danger, personal struggles, and self-discovery.

Struggling with her own fears and desires, Casey hovers between being an innocent teenage girl confused about love and her own sexuality, and becoming a ruthless pirate seeking revenge on those who have taken all she cherished away from her. 

Casey strives to win the trust of the Twenty-One Butterflies, a group of female pirates living on Butterfly Islands, a mystic place filled with dazzling butterflies by day and swarms of moths carrying terrifying parasites by night. A disastrous trip by the Twenty-One Butterflies to a floating city in the sky known as God's Mercy, changes everything. While trapped once more in the hands of her former husband-to-be, Casey discovers that there is a traitor in their midst. From that point on, it is a race to save her new friends' lives and to escape from the man who wishes to make the young girl his wife and a slave to his twisted desires.