Casanga, an island off the West Coast of Africa, 1700 AD. Queen Zinga condemns her husband, the King of Casanga, to death.

Their son and his wife seize the King's medallion and escape from the island, but fall into the hands of slave traders. A montage depicts the historical process by which people of African descent came to settle in Britain.

The horrors of the eighteenth-century slave trade are exposed and its abolition in 1838 is recounted. Throughout the prologue the King of Casanga's song and medallion are passed down from generation to generation to John Zinga, shown working on the docks near Tower Bridge in London in the 1930s 'present'.

London-born John Zinga lives in a racially-mixed community with his wife, Ruth. His best friends are also dockers: Monty, who comes from the Caribbean, and Bert Puddick, a Cockney.

Zinga acknowledges Africa as his ancestral home - which he dreams of visiting - by wearing the Casanga medallion and singing fragments of the King's song he was taught as a child. Zinga possesses a "natural", untrained but powerful singing voice.

Zinga is discovered by Gabriel Donezetti, an Italian impresario who successfully transforms the docker into an internationally acclaimed concert singer.

On the opening night of The Black Emperor, an opera based on The Emperor Jones, Zinga is befriended by Sir James Pyrie, an explorer and anthropologist. He informs Zinga that the medallion he wears around his neck has been inherited from the King of Casanga. Zinga discovers he is a direct descendant of the King. He also learns from Pyrie that Casanga is still inhabited.

Abandoning his concert career, and accompanied by Ruth and Monty, Zinga travels to the island to meet his people. To his dismay he finds that they are superstitious, poverty-stricken and under the influence of Idobo, a witch doctor.

Zinga is met with scorn and abuse from the islanders until he bursts into the sacred song he learnt as a child. This persuades the islanders of his royal heritage. Zinga resumes a part-time concert career in order to raise funds to assist his people.