Sally Porter is a drab waitress, divorcee, and loner in the great city. But, though she is unaware of it, she is also four other, quite different people: Nola, the cold independent artist who has a studio in Greenwich Village; Derry, the happy-go-lucky tomboy; Bella, the sexpot with a talent for singing and dancing; and finally Jinx, the hate- filled killer.
Whenever events put too much of a strain on Sally Porter, she feels a headache and a blackout coming on - and a new character takes over. If there is a man to be fascinated, she will become Bella. If there is an intellectual problem, she will become Nola. And - as happens in the opening scene of the novel - if there is a rapist to be dealt with, she becomes the vicious Jinx.
It is the task of the wise and patient psychiatrist, Dr. Roger Ash - a man who nevertheless has severe problems of his own - to deal with this case of multiple personality and, through painstaking therapy, to try to fuse the four disparate personalities into "the fifth Sally."