Arthur Mumsby and his irritating sister Myrtle live with their father Rev Marmaduke Mumsby in Larklight, a huge and rambling house travelling through space on a remote orbit far beyond the moon. One ordinary sort of morning, they receive correspondence informing them that a gentleman is on his way to visit, a Mr Webster. Visitors to Larklight are rare, and a frenzy of preparation ensues. But it is the wrong sort of preparation, as they discover when their guest arrives, and a dreadful, terrifying and marvellous adventure begins...