Saoirse is visiting her grandad in rural Mayo when he tells her the story of a legendary pirate queen’s lost treasure, supposedly hidden in nearby Clew Bay. Inspired to search for it, she and Grandad’s dog Nessa take out the rowing boat and have an adventure around the islands, encountering all kinds of creatures on their travels, including a magical giant, who just might know exactly where the treasure is hidden. When Saoirse wakes up back in her bed the next morning, she thinks it may all have been a dream – but a mysterious souvenir suggests otherwise ...