Twins Ben and Sophie are excited to be going on a family vacation to the Cayman Islands during their two-week winter break-even though the dark shadow of their dad's alcoholism hangs over them. But it's their eleventh birthday, and they're going to have fun no matter what. Meanwhile, their mother, Kate, struggles to keep it together, even though she knows something has to change.

When the twins' dad, Sam, yells at them on the morning of their birthday, Sophie and Ben decide to take their daysailer, The Sea Jewel, out for a spin. But they quickly find themselves in trouble out at sea, and the boat capsizes in a storm. What happens next sends the children on a magical undersea adventure; they meet a variety of angelic sea creatures who enlighten the twins on their journey of self-discovery. As they swim through shipwrecks, caves, and tunnels, by reefs, walls, and the sea animals of Grand Cayman, Ben and Sophie face their fears, look beyond appearances, and search for the hidden treasure.