 Protagonist Art Mumby narrates an attack on the British Empire and the solar system at large by an ancient, arachnid-like extraterrestrial race, against which he and his family play a central role, aided by the pirate Jack Havock and his crew.The story begins at Larklight, a house that orbits Earth's moon, where the Mumbys receive a visitor from the Royal Xenological Society, a Mr. Webster, who is revealed to be an extraterrestrial resembling an enormous white spider.Art and his sister Myrtle escape; but their father is captured and held prisoner.