By chance, he is informed that an ancestral medallion he wears proves his lineage to African kings, and he leaves fame and fortune behind to take his rightful place as royalty.
The ceremonies were presided over by future first Prime Minister of independent Ghana, Kwame Nkrumah, Robeson's friend from his London years.
The first part of the film's story takes place in the year 1700 on an island called Casanga off the west coast of Africa.
The island has not yet attracted the attention of the slave traders on the mainland, but its people are suffering fierce oppression under their hereditary queen Zinga - a tyrant, despot, and mistress of cruelty.
What first appears on screen is a grassland landscape in which men of an African tribe are patrolling, with haystacks and rugged hills in the background.
Next we see the brutal Queen Zinga, wearing a leopard-skin dress, a straw hat, and a shell necklace on which hangs a medallion, a symbol of kingship.
Suddenly, a girl, apparently frustrated by the cruelty of the planned execution, rushes towards the queen, grabs the medallion, and runs off.
Chains and handcuffs are destroyed, ropes and whips are burned, and a seemingly brand new story begins, centring on John Zinga, a black dockworker in England with a great baritone singing voice.
"No more docker, but a great career" Donizetti promises Zinga, who is shocked and surprised since he thinks he can achieve his dream of travelling back to Africa by this chance.
In the end they find the way to Donizetti's room and he starts a series of tests and introduces several skills to help Zinga improve his singing.
But after his wife's comforting, mainly focussing on the chance of travelling back to Africa, he is persuaded to follow those doctrinaire rules and techniques.
Not good at public speaking, he sings an old song derived from his long lost childhood memory that he barely remembers and into which has to put some words.
At this time Donizetti happens to come in and tells John some good news - a new contract to work in the great New York City.
Even though they have the same skin colour and Zinga has the medallion which proves him the king, he and his fellows are still considered out-siders, strangers or even bad guys coming from the whites’ country.
The witch doctor also announces that when a person gets sick - no matter a fever or cancer - he's dead rather than ill. To deal with this inhuman act, Zinga gives the patient medicine and tries to heal them.
But that violates the taboo of not letting women join the ritual and the witch doctor feels perfectly justified to put Zinga's wife in the basement.
The witch doctor performs different kinds of tricks, such as fire swallowing to intimidate his people, so that Zinga's followers are afraid to rescue them.
Both Zinga and his wife are tied to a wooden pole, waiting to be killed, just as hundreds of years ago, the evil in charge of the island won against the justice.
At the end of the movie, Zinga picks up his career as a singer again and performs the song he sang a long time ago, "Lonely Road" on a stage decorated as his little island, referring to his achieving his dream and the help he brought to his people, leading them to a better life.
Song of Freedom may have been the opportunity Robeson was looking for to (in his words) "give a true picture of many aspects of the life of the coloured man in the West.
Writing for The Spectator in 1936, Graham Greene gave the film a mixed review, describing the direction as "distinguished but not above reproach" and noting that "the story is sentimental and absurd, and yet a sense stays in the memory of an unsophisticated mind fumbling on the edge of simple and popular poetry".