Wide Sargasso Sea by Jean Rhys is generally considered to be a prequel to Jane Eyre. Though the primary characters in this novel also appear in Charlotte Bronte's work, Rhys does take some liberties with the original and her novel is therefore considered a revisionist work.
Revisionist, of course, means it was written with a change. In this case, one of the changes reflects something which has become known as post-colonialism. When Rhys retells the story of Bertha Rochester (Antoinette Mason in Sargasso) and her husband Edward Rochester (her unnamed husband in Sargasso), she highlights the negative impact of European colonization on the native Caribbean culture. By doing this, Rhys clearly attempts to set the record straight about the negative effects of, in this case, British colonization--a direct discounting of the version consistently promulgated by those British colonials who claimed they were helping the natives by their presence.
She presents this revisionist history through the perspective of the black Creole characters in her novel who feel oppressed and abused by the whites who have come to their island and tried to transform the lush, tropical jungle into a proper English settlement. There is no question that the sympathetic characters here are those who have been stifled and oppressed by the white, European encroachers, and there is no question that this portrayal is deliberate. In fact, the timeline of the original novel has been changed by thirty years in order for Rhys to make her revisionist statement in this novel. Both the black natives and the white Creoles, such as Antoinette and her mother, suffer torments at the hands of the white men in their lives.
As a modernist novel, Sargasso is also considered a modernist novel because it reflects the modernist themes of anxiety, isolation in some form, loss, dependence, and a sense of chaos. One way Rhys demonstrates these themes is through the contrast in setting between the lush, sensual, native way of life in Jamaica with the rather cold and calculating mannerisms and lifestyle of the English encroachers/oppressors.
In one way, Rhys takes the tragic but antagonistic figure of Antoinette from Jane Eyre and turns her into a sympathetic victim due to the isolation and confusion caused directly by British colonialization of the island. We understand her extreme isolation due to her ethnicity, her class, and the white men in her life who oppress her. She says:
The blacks hated us, they called us white cockroaches.
This is oppressive, and we see her demise in Jane Eyre as a desperate attempt to connect to something from her pre-colonialism past.
In another way, Rhy takes Antoinette, the helpless victim of her oppressive and heartless husband in Jane Eyre, and turns her into a rather powerful heroine who does what she can to take control of her life in the face of all oppression.
Perhaps a general statement which might encapsulate these two qualities of the novel is that Rhys has taken a familiar, well loved story and created something entirely new from it. Though the characters and setting are certainly recognizable from the original from which it is derived, Rhy has written a new commentary on history and the effects of both isolation and colonialism. She has also used her own language and style rather than trying to mimic the original.
While Sargasso is unquestionably a derivation of Jane Eyre, it is not a lesser work in terms of its social, literary, and historical commentary.