The book deals with the thoughts and psychological transformations of Doctor Fridolin over a two-day period after his wife confesses having had sexual fantasies involving another man.

This culminates in the masquerade ball, an event of masked identity, sex, and danger for Doctor Fridolin, the outsider.

The protagonist of the story is Fridolin, a successful 35-year-old doctor who lives with his wife, Albertina (also translated as Albertine), and their young daughter.

One night, Albertina confesses that the previous summer, while they were on vacation in Denmark, she had a sexual fantasy about a young Danish military officer.

He encounters his old friend Nachtigall, who tells Fridolin that he will be playing piano at a secret high-society orgy that night.

Upon his return home, Albertina awakens and describes a dream she has had: While making love to the Danish officer from her sexual fantasies, she had watched without sympathy as Fridolin was tortured and crucified before her eyes.

He finds his way back to where the orgy had taken place the previous night; before he can enter, he is handed a note addressed to him by name that warns him not to pursue the matter.