Citizen of the Galaxy takes place far in the future, when the human race has spread out to colonize other planets. In a slave market in the capital of Jubbul and the Nine Worlds, an auctioneer announces, "Lot 97. A boy." Slavery is commonplace in Jubbul, and the sight of the ragged, starving boy, Thorby, on the auction block is not unusual. What does puzzle bystanders and Thorby himself is his purchase by crippled Baslim, the beggar who sits every day in a corner of the marketplace.
Thorby soon discovers that Baslim is no ordinary beggar. A master of languages and a superb teacher, Baslim leads a mysterious undercover life that brings Thorby eventually to his own adventure on the Free Trading starship Sisu and finally to the truth about Baslim's identity and his own.