When Arthur and Rose were little, they were the heroes of Roar, a magical world they invented where the wildest creations of their imaginations roamed. Now that they're eleven, Roar is just a distant memory. But it hasn't forgotten them. When their grandfather is spirited away into Roar by the villain who still haunts their nightmares, Arthur and Rose must go back to the world they'd almost left behind. And when they get there, they discover that Grandad isn't the only one who needs their help.