The year is 1888, and Miss Mary Morstan finds herself at the centre of a mystery seemingly too complicated to unravel. Following the disappearance of her father, she is sent each year an anonymous gift of a beautiful pearl. When she receives a mysterious message to meet her unknown benefactor, she seeks out the help of Sherlock Holmes and Dr Watson, and there follows an exotic tale of treasure, intrigue, betrayal and murder. The great detective leads his companions on a wild chase for answers as they hunt down a killer and try to unravel the truth behind the Sign of Four.