On April 19th…

April 19th 1888
Holmes, Watson and Athelney Jones chase the Launch Aurora down the Thames (SIGN)

Tonga was shot by Holmes or Watson, or both.
Watson finds the true treasure of the tale, his beloved Mary.

An ode added by Chips in honor of beloved Mary:

Limerick to Sign of the Four
by Isaac Asimov, BSI and Sherlockian extraordinaire

Muttered Sherlock, “Never mind Cocaine’s pleasure,
Let us seek the famed Agra Treasure.”
Answered Watson, “No pearls,
for myself only girls,
And its Mary that’s made to my measure.”