The Octonauts travel for Christmas to Professor Inkling's childhood home, a seamount, but soon get involved in a rescue mission when a rockslide endangers a rare 500-year-old golden coral and traps Professor Inkling's nephew Squirt! They also learn about Christmas tree worms which live in holes in rocks. Crucial to completing the mission is the new heavy-duty vehicle GUP-X which Tweak has built as an early present for Captain Barnacles.