A mysterious client arrives at 221B Baker Street, initiating a complex investigation into a long-lost treasure and a series of unexplained deaths. Dr. John Watson and Sherlock Holmes are drawn into a case involving Mary Morstan, whose father vanished years prior under suspicious circumstances. The duo must navigate the fog-drenched streets of London and the intricacies of colonial history to track a elusive figure known as the Sign of the Four. The narrative is presented through Watson's first-person perspective, documenting the logical deduction and forensic methods employed by Holmes to solve the puzzle.

Readers and critics frequently highlight this work for its pivotal role in establishing the procedural elements of the detective genre. Discussion often centers on the contrast between Watson's moral observations and Holmes's detached, analytical approach to human behavior. The atmosphere of late 19th-century London is noted for its effectiveness in grounding the high-stakes mystery in a tangible, gritty reality. Many observers point to the exploration of imperial themes as a significant layer that distinguishes this entry from other cases in the canon. The balance between character development and the mechanics of the investigation remains a primary focus for those analyzing the evolution of the series.