In post-war Britain, widower Bert Mallinson raises his teen-aged daughter Betty. He works nights as a signalman at a Southern Railway switch box in the port. During one of his shifts, after the arrival of a freighter in the harbor, he notices a man on the freighter who tosses a small parcel to another man on the pier. He also notices a third man tailing the man with the parcel. After dark, Bert Mallinson witnesses a fight between the two said men over the parcel. One of them kills the other who falls into the harbor with the parcel. The other man flees. Bert dives into the harbor and retrieves the parcel. Upon inspection, Bert discovers 5000 British Pounds in notes inside the parcel. His first thought is to surrender this money to the police but various subsequent events keep him from going to the police. His personal financial needs push him to delay his honest intention. He reluctantly decides to keep the money for himself. However, things get more and more complicated as Bert tries to keep these facts secret from everyone, including his daughter. Also, the man who murdered the other for the parcel returns to the area looking for the money bundle. He goes by the alias Jim Brown. To make matters worse, the police discovers the body of the murdered man floating in the harbor. Their investigation leads them closer to Bert as a possible murder witness. They are assisted in their investigation by French Police Inspector Dupré as the missing loot seems to have originated in France, from a casino heist. Later, Bert falls for a shrewd French dancer named Camelia who ingratiates herself with Bert and his daughter and becomes privy to Bert's secret regarding the money parcel. With so many interested parties searching for the missing money, Bert's personal safety and freedom are at stake.