A dazzling flash cuts through the darkness and with a lightning-fast motion, they take off. Arthur and Monty Brightstorm are two simple, yet so special children. They may not have parents, their home may have been taken from them unfairly, but they have each other and found a new family aboard a skyship. On the other hand, at thirteen years old, they have already traveled to the farthest point ever reached by humans in the Open Sky, the icy Southern Pole, driven by the desire to discover the truth and restore their family's name. Monty is a realist and conscientious – one of the things that makes her an excellent engineer – while Arthur, a very clever boy and true book lover, is used to doing things without thinking. Sometimes, this leads the twins into dangerous situations; like now, for example, when they are lurking on the streets of London, chasing a tall figure slipping through the night like a knife in water.

Arthur spotted him just ten minutes ago, gazing thoughtfully through the window of his room on Archangels Street during the quiet night. The man was coming from the south and heading north, nervously looking around and holding something small in his hands. His behavior seemed so suspicious that Arthur decided to follow him and find out what this man was doing out so late. Monty followed him, more to prevent Arthur from doing something worse. The man stopped and looked back. Monty suddenly pulled Arthur behind a tall wall. "What is he up to?" she whispered.