Torn from their lives as regular Librarian's kids, Max, Ingrid, and Luke must survive in a hidden Librarian outpost--underwater! While their parents scramble to safeguard the books of the Overworld, the trio sneak out of their warren of underwater caves to investigate the deep waters of the oceans. When they hear about the Legend of the Coral Caves, a trove of golden coral beyond imagining, Max, Luke, and Ingrid realize that this wealth could help solve all their problems. Now it's up to them to find the cave and bring the fortune back to save their families--and their books!