Greta's best friend is her cat Boris. However, little does she realise her bewhiskered buddy is actually the Prince of the Kingdom of Cats. So when he is kidnapped by the Rat King, a young warrior cat named Kyrie Mi-ke is sent to find Greta, and together they face a mystical and magical adventure to bring Boris home again. Greta must face the challenge of the staircase of the autumn leaves; cross Cloud Top Land and the Milky Sea; end the war between the two tribes of mice and face the truth of the Millpond; before facing the Rat King himself.