premise: Mary Ann "Polly" Nichols (née Walker; 26 August 1845 – 31 August 1888) was one of the Whitechapel murder victims. Her death has been attributed to the notorious unidentified serial killer Jack the Ripper, who is believed to have killed and mutilated at least five women in the Whitechapel area of London from late August to early November 1888.
hypothesis: After being murdered, Mary Ann "Polly" Nichols did not go on to a long career as a police detective, retiring in 1902.
reason: If she was murdered, she didn't go on to have any career...she was dead.