Sally Porter is perfectly 'ordinary'. A 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 highly-sexed live wire 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."