The Sign of Four follows the famous detective Sherlock Holmes and his friend Dr. Watson as they investigate a mysterious case brought by Mary Morstan. She has been receiving valuable pearls from an unknown sender and seeks help to uncover the truth.
The investigation leads Holmes and Watson into a complex story involving a hidden treasure, betrayal, and a secret pact known as the Sign of Four. As they follow the clues across London, they face danger and uncover a past crime connected to India.
In the end, the mystery is solved, the criminals are revealed, and justice is served, while Watson develops a romantic relationship with Mary.