The story takes place in England in 1947. The famous detective Sherlock Holmes (Ian McKellen) is now a 93-year-old man. In his later years, he lived a solitary life, accompanied by his side only the housekeeper Mrs. Mang Luo (Laura Linney) and her son Roger (Milo Parker). Although he is no longer concerned about the various disturbances in the world, there are still some unsolved mysteries for Holmes' wonderful life that make him feel bothered. The increasingly stiff body, the deteriorating memory and reasoning ability make him more and more aware that now, at this moment, is his last chance to solve these mysteries. In this way, in the absence of his old partner Watson, Holmes began his last reasoning alone.