The 1947 film "Johnny O'Clock" is about several people involved with a large gambling casino. Johnny O'Clock, a dapper young man, is part-owner of the casino. His partner is Guido Marchettis, an older businessman, who is married to Nelle, a "trophy wife." Harriet Hobson works for the casino and wanted to be Johnny's girlfriend. When Harriet is killed, her sister Nancy Hobson arrives to handle the arrangements and soon is als attracted to Johnny and begins seeing him. Police Inspector Koch arrives to investigate Harriet's death and the disappearance of a crooked policeman named Blayden. Johnny finds a Mexican silver peso in an apartment belonging to the dead woman which points to the woman's killer. The coin found is an old coin, not a then current Mexican coin.

Police Inspector Koch visits a large hotel looking for Johnny O'Clock.

Koch explains to the hotel clerk that Johnny has many names, this is one of the more unusual ones. The clerk gives Koch the room number and Koch heads up to the room. Johnny's room has a table with some loose change on it.

The coins on the table appear to be Washington quarters (25-cent coins). Koch finds Johnny with Harriet, a casino employee.

Koch questions Johnny about a crooked policeman Blayden who was killed after he killed a gambler. Harriet had been Blayden's girlfriend but he had dropped her and she was taking the breakup badly. Later Johnny heads to the casino which he is part-owner of to check things out.

There are no bills or coins on the tables, only chips. Koch goes to question Harriet and finds her dead from a supposed gas leak suicide. Nancy Hobson, Harriet's sister, arrives and meets Johnny.

Johnny and Nancy become close friends. Johnny's partner in the casino is Guido Marchettis, a long time gangster. Guido has returned from a trip to Mexico which he constantly talks about. He shows around a portrait of himself painted by a Mexican artist.

Guido shows everyone the picture. Guido's wife Nelle Marchettis likes to drink. At one time she and Johnny had an affair, he dropped her, but she still loves him. Inspector Koch has received a report from a medical examiner that Harriet's death was not a suicide but a murder, and later Johnny also learns the fact. Nancy heads to her sister's apartment to get some things and meets Johnny who is searching the place. Johnny finds a large Mexican silver peso on the carpet.

The coin is a Mexican silver peso of the type issued by several Mexican mints from 1898 to 1909. These coins were the same size as the American silver dollars.

A Mexico peso of this type dated 1898:

Silver, 39 mm, 27.03 gm, Mexico City mint
Obverse: Liberty Cap and rays
Reverse: Eagle on cactus holding serpent in beak

Johnny picks up the peso and in the dark room the coin shines.

Johnny takes the coin with him.

Johnny shows Koch the Mexican coin and talks about Guido's recent trip to Mexico. Koch and Johnny believe that Guido killed the crooked policeman and later killed Harriet because she knew about the policeman's death. Johnny and Nancy have fallen in love and while driving together someone fires a gun at him. Johnny believes that Guido is behind the shooting and and decides to sever their relationship. Johnny visits Guido at the casino to get his partnership money. Guido opens a safe and hands Johnny a large amount of cash, then both men pull out guns and shoot each other. Guido is killed and Johnny is injured. Nancy arrives and stays with Johnny while they wait for Koch to appear and find Guido.

Nancy convinces Johnny to go with Koch to the police station.

He gets the girl, too.

Dick Powell as Johnny O'Clock
Evelyn Keyes as Nancy Hobson
Lee J. Cobb as Inspector Koch
Nina Foch as Harriet Hobson
Thomas Gomez as Guido Marchettis
Ellen Drew as Nelle Marchettis
Director: Robert Rossen
Writers: Robert Rossen, Milton Holmes