The trilogy revolves around former residents of the small village of Deptford, Ontario (as the Salterton trilogy does around the village of Salterton, and the Cornish around the Cornish family). Davies takes three different viewpoints in each of the novels, and approaches each of the novels in different styles.
Each of Deptford's refugees has come by twisting paths from their simple village - and each carries a secret that crosses the lives of the others and drives the plot forward. The greatest secret is one that we are not even aware of until the close of the last book, but knowing it finally answers questions about the relationships of several major characters.
The protagonist of Fifth Business, Dunstan Ramsey, crops up in several other novels by Davies (other than appearing as a major character of the other two books in the trilogy). He's a gentle schoolmaster with surprising depths and is probably the stand-in for Davies himself. He counsels his students to write in "the plain style", as Davies does - to highlight the story rather than the writer.