The story takes place in England in 1884. Inola (Millie Bobby Brown) is preparing to celebrate her 16th birthday. On this day, her mother (Helena Bonham Carter) disappears. The famous detectives Charlotte Holmes (Henry Cavill) and McCoff (Sam Claflin) become Inola's guardians. Faced with her quirky and uncontrolled sister, the two men wanted to send her to a boarding school that specializes in training "ladies". How could Inola be so obedient and be manipulated? After a disagreement, she chose to run away from home and went to London alone to find her mother's whereabouts. During the journey, Inola was involved in a very mysterious case. She turned into a detective and started a battle of wits and courage with her brother.