Robert Montgomery stars with Elizabeth Allen and Lewis Stone in "The Mysterious Mr. X" from 1934.
A serial killer is going around London killing policemen. Unfortunately for Nick Revel and his band of merry men, a taxi driver named Joe and an insurance clerk named Hutch, the killer struck outside a home where the famous Drayton Diamond was stolen. Nick is the thief. But he's not the killer known as "Mr. X," who writes letters to the police just before he strikes. The police think the robber is the killer.
Nick and his cronies know that they can't return the diamond for the insurance reward or pawn it until the serial killer is caught. When a man is arrested, one Sir Christopher March, Nick knows he's the wrong man. He has an idea as to how the police can catch the killer. He figures if he testifies on behalf of March, he will be able to present his plan to the police.
March turns out to be the fiancé of Jane, the daughter of the police commissioner, and Revel and Jane are attracted to one another. Meanwhile, her father is suspicious of Revel.
Pleasant mystery, with a charming performance by Montgomery. Some fun moments, especially when an accomplice delivers the diamond to Revel and drops it in a glass of liquid. The end is quite exciting, with a very dynamic either set or location.
Good movie.