Ruby Halliday is surprised to find that her great-grandmother has left her a pearl necklace and a black cat in her Will. She is even more astonished to find that the cat can talk, and the ghost of her great-grandmother has appeared to help her.
For a blue ruby and two black pearls are missing from the necklace. Now Ruby must complete a desperate quest to find them, travelling under the sea and back in time.

Ruby meets Scarlett and they travel to Africa to find a red diamond, stolen by the Aztec Queen of the Underworld. Back at school, Ruby and Scarlett are shocked when the Aztec Queen returns to look for the diamond, and then they find that new girl Emerald Black is not all she seems. Now the magic really begins...