Past Master is set in the year 2535 on the world of Astrobe, a utopian Earth colony that is hailed as Golden Astrobe, "mankind's third chance", after the decline of both the Old World and New World on Earth.
In an attempt to save their dying civilization, its leaders use time travel to fetch Sir Thomas More (chosen for his fine legal and moral sense) from shortly before his death in the year 1535 to be the president of Astrobe.