Harry Angel had fought in World War II, but was sent home after suffering from severe injuries and shellshock. When he was released from the hospital some time later, he managed to start making a living working as a private detective.
After spending twelve years in Los Angeles, Angel met up with a mysterious client by the name of Louis Cypher, who hired him to find a missing person by the name of Johnny Favorite. Angel reluctantly took the case and started his investigations by visiting the hospital where Favoritewas last seen after returning from fighting in the War.
Unfortunately, when Angel tried to speak to the doctor that had looked after Favorite, he found him mysteriously killed, shot through the eye. One by one many of the other people associated with Favorite's past were inexplicably murdered, and every time it made Favorite a larger suspect for the police.
Finally, Angel managed to discover the horrific truth behind everything- specifically that Louis Cypher was Lucifer, and that he himself was Favorite, and had killed everybody. Harry quickly ran back to his apartment only to find his girlfriend, who he now knows was his daughter, to have been murdered, and he was quickly arrested. Angel was presumably executed by electric chair for the murders as the film ends with him descending into Hell, thus fulfilling his debt to Cypher.