In this book, Arthur (Art) Mumsby and his irritating sister Myrtle live with their father Revd Marmaduke Mumsby in the huge and rambling house, Larklight, travelling through space on a remote orbit far beyond the moon. One ordinary sort of morning they receive a correspondence informing them that a gentleman is on his way to visit, a Mr. Webster. Visitors to Larklight are rare if not unique, and a frenzy of preparation ensues. But it is the wrong sort of preparation, as they discover when their guest arrives, and a Dreadful and Terrifying (and marvellous) adventure begins. It takes them to the furthest reaches of known space, where they must battle the evil First Ones in a desperate attempt to save each other - and the universe.