Joe Friday and Frank Smith are working the nightwatch in the Homacide Department. They respond to a suicide call. After entering the locked room where the body is located, Friday interviews the man’s wife to get information, and details about the dead man. Then the mother in law is interviewed. A portrait of a broken marriage, and of the man’s life is brought out, but nothing to doubt a suicide was committed.
The lab later finds a discrepancy with the bullet that killed the man, and it becomes clear that the suicide is really a murder. The bullet is of a different calibre than the gun he held in his hand. Now the gun needs to be found, so the crime scene is reevaluated. The stories of the witnesses are reevaluated, and the mother in law breaks down and admits she did it.