The 1987 film "The Sign of Four" is based on the 1890 Arthur Conan Doyle novel The Sign of Four, also titled The Sign of the Four. The story features his famous detective Sherlock Holmes and his partner Dr. John Watson. A young woman asks Holmes and Watson for help in locating her father, a British Army officer who returned from duty in India. Holmes finds out that the officer and another officer were involved in transporting a large cache of jewels from India to England. At one point Holmes pays some boys with shillings and one with a guinea for information. Later a man involved with the treasure describes the two officers gambling for what appear to be Arabic coins.

Miss Mary Morstan appears at the Holmes and Watson residence at 221B Baker Street in London.

She tells Holmes that her father Captain Morstan returned to Britain from duty in the Andaman Islands near India, contacted her, and then disappeared. She continues that his friend, Major Sholto, had already returned to London. She then explains that someone has been sending her valuable pearls each year and recently sent her a note asking her to meet a man at a theatre.

Holmes has been bored with life and takes the case for excitement. He and Watson accompany Mary to the theatre where they are met by a coachman who takes them to the large house of Bartholomew Sholto, one of two twin sons of the deceased Major Sholto.

Bartholomew tells the three that Captain Morstan visited Major Sholto, they argued about the treasure, and the captain collapsed and died. The major had his servants bury the body. Bartholomew explains that his brother Thaddeus has the treasure in a box in his attic and takes the three to his brother's large house. There they find the brother dead and the box removed. Holmes sends a message to the police and Inspector Athelney Jones and some policemen come. Holmes finds that Thaddeus was killed by a poisoned blow-gun dart and finds footprints of a one-legged man and a very small man.

There is also a note on the table in front of Thaddeus which reads "The Sign of Four". Holmes concludes that the one-legged man was a Jonathan Small who had signed another note. The two Army officers were in charge of a British prison in the Andaman Islands and Small had been a prisoner there who escaped with a native man and returned to England.

The Inspector has Thaddeus and a servant arrested. Holmes has an arrangement with a group of boys called the "Baker Street Irregulars" who perform small tasks for him for pay.

Holmes gives Wiggins, the group's leader, a shilling.

He asks the group to look for a one-legged man accompanied by a smaller person and gives each boy some money.

Holmes promises a "guinea" to the boy who finds Small. A guinea was officially a gold coin worth around a pound which was last minted in 1814 and was replaced by the gold sovereign coin. However the term "guinea" lasted a lot longer and turns up in literature even in the 1930's. The coins land on the table.

The boys grab the coins and leave. Later Wiggins reports that they have located Small and that he has chartered a boat to take him to the docks. Holmes and Watson request that Inspector Jones obtain a police boat and they set out to chase Small's boat. Tonga, the Andaman Islands native, is the little man accompaning Small. He carries a blow gun which fires poison darts.

Holmes shoots Tonga who falls into the water. The police boat overtakes Small's boat and he is captured and taken to Holmes's residence.

Jonathan is given a drink and then tells his story.

Jonathan had joined the British Army in the 1850's and was sent to India. He had an accident and lost his leg. He was then discharged as unfit and went to work on a plantation near Agra. In 1857 the Indian Mutiny broke out and he fled to the Agra Fort where he was put to work as a guard. Two Sikh guards grabbed Jonathan and told him that he could join them in a robbery or be immediately killed. He decided to join the robbery of a courier bringing a box of jewels belonging to a Rajah and being carried by an employee of the Rajah. A third Sikh, also a member of the robbery group, is with him.

The Sikhs and Jonathan kill the man and open the box.

They bury the box and the dead man in the fort. The British suppress the Mutiny and then the body is discovered. The three Sikhs and Small are sent to the Andaman Islands prison. Small has managed to get a job as an assistant to the prison doctor and he sees that the two Army officer Morstan and Sholto have been losing money at cards. Jonathan imagines himself looking at the money.

The coins appear to be thin silver coins with Arabic writing on them.

Jonathan talks to the three Sikhs and they decide to invite the two officers to share in the treasure in exchange for their help in gaining freedom for the four men known as the "Sign of Four".

Small, the two officers, and the three Sikhs sign an agreement to share the treasure.

Major Sholto returned to England with the treasure but did nothing to assist the four. Later Captain Morstan returned to England and contacted the major where he died. Jonathan Small concludes his story and the men open the box.

Jonathan laughingly explains that when his friend Tonga was killed he threw the contents of the box into the Thames river. Mary gets ready to leave.

Dr. Watson is sorry to see her go. Holmes has no interest in her and goes to bed.

Jeremy Brett as Sherlock Holmes
Edward Hardwicke as Dr. John Watson
Jenny Seagrove as Miss Mary Morstan
John Thaw as Jonathan Small
Kiran Shah as Tonga
Ronald Lacey as Thaddeus and Bartholomew Sholto
Emrys James as Inspector Athelney Jones
Director: Peter Hammond
Writers: John Hawkesworth, Arthur Conan Doyle (novel)