"Dressed to Kill" is a psychological thriller and murder mystery. Kate Miller (Angie Dickenson) is a frustrated housewife in an unhappy marriage. Her therapist, Doctor Robert Elliot (Michael Caine), isn't really helping her adequately resolve the situation. She leaves his office and makes a spur of the moment bad decision to go home with a total stranger. Just as it seems her life is about to careen out of control due to this poor decision, she ends up gruesomely murdered by a tall blonde woman.
Liz Blake (Nancy Allen) stumbles upon the body immediately afterwards, thus also catching a fleeting glimpse of the murderer. This makes her the only witness. She has a shady profession, which helps make her the prime suspect as well. The police do not really buy her bizarre story. Kate's son Peter Miller (Keith Gordon) soon becomes Liz's only ally, as he tries to both protect her from the real murderer and find clues to who really did it.
They soon begin to suspect that the murderer must be one of Dr. Elliot's patients. Of course, they cannot ask him due to doctor patient confidentiality rules. So they begin filming who enters and leaves his office and trying to find out information various other ways. The last person to leave for the day is a tall blonde woman who could be the murderer. Liz and Peter are determined to find out who the last patient is for that day.
Just about the time Liz discovers she has stumbled into danger and is alone with the murderer, Peter realizes no one left after the blonde woman. In other words, Dr. Elliot was never filmed leaving his own office. Peter realizes Dr. Elliot and the female murderer are one and the same person. Fortunately, there is another tall, blonde woman on the scene as well: A police woman assigned to protect Liz. She shoots Dr. Elliot to protect Liz and then arrests him.