Professor Moriaty: Are you sure you want to play this game?
Sherlock Holmes: I'm afraid you'd lose.
Sherlock looks at the ring of a murdered man, who was killed in a blast along with about a dozen other men in Paris, and the ring was plain. But later, the ring is shown again, and this time it has the letter M on it, a large letter, and it is very ornate - nothing like the other ring.
The plot of the movie closely follows the short story "The Final Problem", which is included in "The Memoirs of Sherlock Holmes".