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.