Wide Sargasso Sea is supposed to be a prequel to Jane Eyre. There's not way to compare the two. Jane Eyre is good. Wide Sargasso Sea is terrible. How in the world it is considered a classic and Jean Rhys' finest work is far beyond me.
Wide Sargasso Sea is supposed to be Bertha's story. Remember Bertha? She was the mad woman in the attic in Jane Eyre. In Rhys' vision of Bertha's life, Bertha is not only named wrong, but was wronged her entire life. Bertha, aka Antoinette, grew up poor after her father died. Until the day Mr. Mason came to town.
Wait, let me back up. Antoinette grew up in the Caribbean on an island. She was from one of the few white families there and her ancestors had been slaveowners. The locals hated her family. Her mother was beautiful and proud. And when her husband wronged her, she went crazy and tried to kill him. But "crazy" is relative in this book. It depends on who you ask.
Really, that's all I'm going to say about the plot. It sucked. The writing was so confusing I had to read multiple passages multiple times to figure out what exactly was going on, who was involved, and who exactly the characters were. I read the first page three times to try to decipher exactly who Antoinette was talking about. That doesn't bode well for the rest of the book and it held true.
My recommendation is to skip this book. It does nothing for Jane Eyre or any of the characters involved.