So, you've gone invisible, the end of the world is nigh, and the bad guys have kidnapped your mum (as they do). Who are you going to call? Nope, not those guys (or, in the more recent film, gals) although there are a fair few not-quite-ghosts floating around in this story. In fact, dear readers, your dream team to stop the baddie and save the planet (honestly, the number of times poor old Earth is in danger in stories for young people, it's a wonder we get any sleep at nights) is a Victorian lady inventor, a five-hundred-year-old warrior knight and his trusty steed. Well, actually, it's a pony, but let's not get technical.