Holmes, Watson and Jones enjoy dinner together, talking about a wide range of subjects other than the case. They drink a toast before leaving for the river; Holmes tells Watson to bring his revolver.
There is a celebratory atmosphere based on the presumed resolution of the case—all thanks to Holmes’ work. The dinner table discussion displays Holmes’ intellect, painting him as the quintessential Victorian gentleman.
The three men board the police boat and head out in search of the Aurora. Holmes explains that, while conducting his chemical experiment during the night, it had occurred to him that Jonathan Small would want to leave England under the cover of darkness. He reasons that Small would have wanted to lay low until he knew it was safe to leave, meaning that the ship is probably still nearby—possibly with a boat-builder or repairer, thereby being hidden from view but readily accessible when the moment came.
Holmes displays his genius powers of logic. Essentially, he creates a causal chain of the likeliest events and, of course, for the purpose of the stories, is almost always right.
Holmes had then made inquiries at various shipyards along the river before chancing on one where the foreman had recently taken a ship from a wooden-legged man. At that very moment, Mordecai Smith had come to instruct the foreman that he would need the boat at 8 P.M. that evening. Holmes has stationed one of the Baker Street Irregulars at the yard to wave to them when the Aurora takes off.
The wooden leg acted as confirmation that Holmes had found the right place. The name of the ship, which roughly means “dawn,” might be read as a gesture to the “dawning” of the solution in Holmes’ mind.
The boat goes along the river and stops near Jacobson’s Yard, where Holmes tracked the Aurora to. They notice the signal—the waving handkerchief. The Aurora leaves the shipyard; Holmes and the others give pursuit. Holmes shouts at the men operating their boat to “pile it on!”
Jacobson’s Yard is a fictitious shipyard positioned near Tower Bridge. The handkerchief is subtly comic detail by Doyle, given that people would often wave ships away in a similar manner. The boat chase set up is intended to excite the reader and build the story towards its climax.
They pursue the Aurora down the river, dodging past other boats. Watson notices the figures on the deck, including “a dark mass, which looked like a Newfoundland dog.” They yell at the other boat to stop; a wooden-legged man shakes his fists at them, cursing.
The “dark mass” is Tonga, who is consistently characterized as being sub-human, directly compared here to a dog. The sighting of the wooden leg confirms Holmes’ hypothesis about the case and therefore also validates his ingenious command of logical thinking.
The “dark mass” turns out to be “a little black man […] with a great, misshapen head and a shock of tangled, disheveled hair.” Watson says that “never have I seen features so deeply marked with all bestiality and cruelty […] his thick lips were writhed back from his teeth, which grinned and chattered at us with half-animal fury.” When Holmes and Watson notice the man (later revealed as Tonga) pulling a short piece of wood to his lips, they fire their pistols at him, causing him to fall into the water.
The descriptions of Tonga are clearly racist and link with the idea that black people are inferior to white people. Tonga is once again characterized as “animal” rather than human. Tonga dies here, having been a prominent part of the novel without once having any of his own input. This means he represents the ultimate “other,” never brought into the close circle of the plot’s development and embodying the fear of the other. His weapon is no match for the more advanced Western pistols, subconsciously echoing the means by which the British Empire was able to extend such a vast network of power across the world (which was also highly dependent on the psychology of Imperialism).
The Aurora runs aground on the shore in “wild and desolate” marsh-land. Holmes and his company ensnare Jonathan Small, and recover the treasure chest from the boat. Holmes and Watson notice a poison dart stuck in the wood on their boat; they are relieved to have escaped death.
The main drama of the novella is thus brought to a close, with Tonga, the accomplice but primary evil character, vanquished, and the treasure recovered. With that, Holmes’ rational powers prove to be triumphant.