The Roman conqueror Julius Caesar (Claude Rains) takes over the Egyptian city of Alexandria, and in so doing inherits a pressing problem: in accordance with local custom, the beautiful young princess Cleopatra (Vivien Leigh) and her even younger brother Ptolemy (Anthony Harvey) have been married and are to rule the country together, despite their competing desires to rule alone. Caesar takes Cleopatra under his wing, mentoring her in the uses of political power.