The Octonauts are planning to spend Christmas with Professor Inkling at his childhood home, an undersea mountain. But when they arrive the Octonauts discover that a rockslide has endangered the lives of the local creatures - including a rare species of golden coral, and Inkling's little octopus nephew, Squirt! With the help of Tweak's latest invention, the amazing Gup X, the Octonauts must rescue all the creatures and save Christmas. Octonauts, let's do this!