"Have you ever seen your own shadow on a sunny day? Objects can make shadows too. Read to learn how light makes shadows and reflections"--Provided by publisher.
The Octonauts use their underwater Octopod base to travel to the mysterious Sea of Shade to discover why all the world's shadows have gone missing and convince the Shade King to return the shadows.
Little Phil Groundhog and his shadow do everything together, but when they grow up the more adventurous shadow wants to travel, so he leaves--but neither is happy, so Phil goes searching for his friend.