The storyline of "The Rayner-Slade Amalgamation" likely revolves around a mystery involving the business dealings of the Rayner-Slade Amalgamation company. As with many of Fletcher's novels, the plot may feature a blend of intricate detective work, suspenseful elements, and unexpected twists as the characters attempt to unravel the mystery.

J. S. Fletcher was recognized for his ability to craft compelling mysteries, create intricate plots, and develop engaging characters. His works often incorporated elements of crime investigation, intricate puzzles, and a sense of intrigue, appealing to fans of classic detective fiction.

While I can provide an overview of Fletcher's style and the expected themes in "The Rayner-Slade Amalgamation," my information might not encompass specific plot details or character analyses beyond the overall context of his works. For an in-depth understanding or specific details about this particular book, consulting the book itself or exploring more recent critiques might provide additional insights.