Returning to his childhood home, Alex finds himself standing beside the duck pond of the old Sussex farmhouse where he used to play. He’s transported to his 11th birthday, spring half term. When his dad was struggling to make ends meet and his friend Lettie claimed it wasn’t a pond, but an ocean… Plunged into a magical world, Alex and Lettie’s survival depends on their ability to reckon with dark, ancient forces that threaten to destroy everything around them.