It is quite hard to give a cohesive summary of A Brighter Sun just because the book is so episodic in nature (in that there is a connecting theme about what Tiger learns, but much of the book can be shuffled and produce the same result). Regardless, I am happy to attempt a general summary for you.
Joe Martin and his wife, Rita, are neighbors with Tiger and his wife, Urmilla, in Barataria (in Trinidad). They seem to exist in two totally different classes. Joe and Rita are middle class with a brick house and running water and electricity. In contrast, Tiger and Urmilla are in the lower class with a mud hut with no water or electricity. Even though they are vastly different in regard to heritage (East Indian vs. Black) and in regard to class (lower class vs. middle class), the couples become good friends. Tiger is expected to be a simple farmer:
You gettam house which side Barataria, gettam land, cowwell, you go live dat side. Haveam plenty boy chile—girl chile no good, only bring trouble on yuh head. You live dat side, plantam garden, live good.
These expectations aren't quite enough for Tiger. As Tiger learns what it means to be a man, he falls in love with his own native Trinidad despite his lower-class status. He has many experiences that provide him an "education" in that he meets others who encourage him to read, he is snubbed by upper-class whites, and other episodic plot points.
Tiger's life changes when the American military arrives in order to build a highway through the Barataria farms. As a result, Tiger gives up a life of being a peasant farmer for being a construction worker (and road builder). Tiger is eager to improve the status for himself and his wife, so he invites two of his American bosses over for a good meal. The Americans are impressed, but ask that many of the traditions of Trinidad be violated. After the dinner, a drunk Tiger beats Urmilla (who is now pregnant with his child).
This incident is the catalyst for a culture lesson for poor Tiger who is unable to marry the concept of many cultures in one place. American, Black, East Indian all seem to muddle together in his mind now. Tiger tries desperately to redeem himself by finding a doctor for Urmilla but, due to his class, is rejected again and again until he finds a doctor of yet another nationality: English.
Regardless, due to Tiger's beating of his wife, Urmilla's baby is stillborn. Tiger is devastated and tries (again) to redeem himself by building a new home better than their hut. With true contrition, Tiger is forgiven both by his wife and his community. Everyone joins Tiger's quest to build this house and the story ends with a note of hope (yet a note of uncertainty) about the future.