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While Irishman Michael Denis, a crime reporter for a London newspaper, stops at his favorite pub before a vacation, barmaid Bessie Beavers and her boyfriend, Bill Hawkins, rush in to report the murder of Bill's employer, Mr. Casey.  Although no body is found, shortly thereafter a constable is shot while investigating an umbrella-carrying man mysteriously dumping something in the river.  The man escapes, but Casey's cap and coat are found nearby.

As Denis leaves the pub, his dog Jones runs away, leading him to a house in which a man with an umbrella is breaking into a window.  The man turns out to be the butler who had forgotten his key, but inside, Denis meets socialite Patricia Herrick, who is impressed with his crime stories and bored with her sedate life as the daughter of Sir Arthur Herrick.

Still determined to go on his vacation, Michael goes back to the pub with police inspector Jefferson, but a rock is thrown through the window with a note saying "Michael Denis keep off this case or be seen no more" written with the same red chalk as a similar note found in Casey's shop.  Deciding to postpone his holiday, Michael searches the shop, where he is met by Patricia, who wants to help him, despite his concerns for her safety.

Later, after he tricks her father into taking her home, a local known as the "Rabbit Man" comes to the pub and says he is moving from the neighborhood.  After he leaves, they discover red chalk prints made by his shoes.  Some time later, while again searching Casey's shop, Michael and Patricia narrowly miss being shot through a peep hole, but the police can find no trace of a gunman.  They discover Sir Arthur's frightened secretary Correy in the fog instead.

Meanwhile, Bessie overhears Casey's friend Van Kranz arguing above the pub and is shot while Bill drunkenly sleeps.  After the shot, Bill sees the "Umbrella Man" rushing from the pub.  Soon the papers' constant reporting of the crime arouses suspicion of every man carrying an umbrella.

Next day, Sir Arthur gets a call from the "Umbrella Man" requesting $10,000 or Patricia will be killed.  Michael discovers that the address to which the money is to be sent does not exist, meaning that the money is to be intercepted en route.  He devises a plan and instructs both Patricia and her father not to tell anyone.  At the appointed hour, Michael and Jefferson are waiting to see what happens with the package of money, but while two policemen investigate an umbrella, the postman is attacked and the package is stolen.  Michael soon deduces that the Umbrella Man is really Von Kranz and Casey in different disguises and later concludes that Correy is really all of the men because Jones, who rarely growls, had growled at Correy, Von Krantz and the Rabbit Man.  As they go to apprehend Correy, he is trying to use Patricia as a hostage, but his plan is thrown off by the appearance of a drunken Bill, whom he shoots.  In the fog, Michael jumps Correy after finding a shoe Patricia dropped, and Correy is killed.

With the case concluded, Michael is ready to go to Paris alone, but Patricia has plans of her own.