Sisters Cas and Ori are among the first children born on Mars. The Generation Mars series follows their adventures as they grow up in an environment unlike any in which children have grown up before. Each book in the series uses a fundamental element of survival as title and jumping off point: Air, Shelter, Water, Food. In book two, Shelter, an expedition on the surface goes terribly wrong. As conditions go from uncomfortable to desperate, it falls to Cas and Ori to save the family. It's a struggle for survival in which they learn the full meaning of shelter.