TCM says:
Bank teller Vince Grayson dreams he is in a mysterious mirror-panelled octagonal room, where a man accompanied by a blonde woman is robbing a safe. Vince and the man fight, and when the man begins to strangle Vince, the woman hands him an awl, with which he pierces the stranger's heart. The woman flees, and Vince places the man's body behind one of the mirrored doors and locks it, taking the key. When Vince awakens, he discovers the key in his coat and thinks that he may be a murderer. Distraught, he calls in sick at work and visits his brother-in-law, homicide detective Cliff Harlan.