A thousand years hence, all the major cities of the world are at sea, floating on huge rafts, using ocean currents to navigate. Sal, a young girl on the Sydney raft, training as a navigator, visits Shanghai which is the largest raft city of all, on holiday. She hopes to find the famous genetic scientist, Jezzy, who will modify her body to give her gills. She wants to be like a mermaid, free to live in the sea with the fish. She does meet the old woman but when she leaves her laboratory is arrested as a deserter and mutineer. She is flown back to her home raft for trial. After a tribunal she is released after all to join Jezzy and like many others, is modifed as she desired. There is a strong public reaction against Jezzy s operations. In fear of being marooned to die on land, Jezzy and her young changelings break away their section of the Shanghai raft away drift off independently. Sal finds a lover and gives birth, but a violent storm damages the raft severely and they struggle to avoid sinking. They can survive only by beaching themselves. Sal, the young navigator, plays an important role in achieving this. The remnants of a land based tribe are encountered but more dangers must be faced from marauding gangs of pirates. The rafters make plans for rebuilding their city and returning to the sea.