Alva and Irva are indentical twin sisters who live in the city of Entralla. Along with Gondal, Brobdingnag and the Emerald City, Entralla is not a place you are likely to have visited; only one guidebook to the place exists, despite its historic landmarks and the considerable civic pride of its inhabitants. Alva, by nature an explorer, longs to travel the world. Irva is a recluse, for whom a step outside the house is an ordeal. But the twins belong together; they cannot survive without each other. Since childhood, they have built fantastical cities of plasticine in an attempt to find a place for themselves in the world -- real or imaginary. But it is only when Irva finally refuses to leave the house at all that the major work of their lives begins. Alva, in an attempt to return Irva to life, brings the city of Entralla into their shared home; she wanders its streets, observing, taking notes, measuring, and reporting her findings to Irva, who painstakingly constructs a miniature Entralla.