#last chapter of this arc
At a harbor in Thames River, London, Sherlock and John was monitoring a steamboat named Aurora from afar.
The hunting dog led them to the harbor, where they found out that someone paid a hefty sum for a boat ride on Aurora, but that person asked to wait on their call before departing. Sherlock deduces that they must be waiting for night to arrive before escaping.
So Sherlock had Lestrade prepare a high speed patrol boat for them. They boarded that boat and waited for Jonathan Small to appear and escape on Aurora.
John questions why they had to chase Small instead of just catching him immediately. Sherlock explains that Small would just target Mary when he gets out of prison again for the treasure. To prevent that, they need to make Small give up on treasure.
Small finally boards Aurora and the boat departs, with Sherlock and company following them. Once they get close to Aurora, Small finally notices them.
Sherlock yells at him, telling him that they know he’s involved with Bartholomew’s murder and how they took the Treasures of Agra.
Small replies that he has no plans on handing over the treasure and gets Tonga, Bartholomew’s murderer, to shoot a poisoned needle at him. John pushes Sherlock and himself out of range, and both of them shoot back at him.
Tonga is shot and dies, falling into the river. Since the boat can’t go any faster, Small realizes there’s no way he can escape and threw away all the contents of the chest into the river. If he can’t get the treasure, he’d rather no one else will.
This is what Sherlock had been aiming for the start. Small gave up on the treasure himself and got captured.
On their way back to Baker Street, John is relieved, saying that Mary is finally safe now, but Sherlock doesn’t seem convinced.
Once they got back, they told Mary what had happened. Mary inquires about the treasure and Sherlock replies that Small threw them all down the river. It was thrown near the wetlands, so it would be difficult to recover any of them without time and help from a lot of people.
From there, Mary breaks down and finally admits that someone threatened her to hand over that treasure, otherwise they would annul her marriage with John.
When asked who it was, Mary refused to say. However, Sherlock already had an idea who it was. He says there is only one person in London who would do such a thing. Charles Augustus Milverton.
