[From back cover]Time travellers: Why is Professor Childermass acting so strange? Johnny Dixon and his friend Fergie decide to sneak over to the professor's house to investigate. What they find is wilder than anything they could have imagined, for the professor leads them through a secret cellar door and onto a rickety old trolley. It's a time machine to yesterday, and soon Johnny and Fergie are off on the ride of their lives to Constantinople during the Turkish invasion of 1453!