Gregor lives with his mother, father, and sister (Grete). One morning, he wakes up and realizes that he has transformed into a bug. As he ponders what to do, his family comes to his door, reminding him that it is time to get up and start his day. Because he does not get out of bed, he is late for work. The chief clerk comes to his house to question why he did not leave on the early train. Through all of this, Gregor's mother insists that Gregor is sick, and the chief clerk insists that, whether sick or not, business and work are priorities.
Finally, Gregor decides to try to get out of bed, although he is a giant bug. He has been talking with with his family and the chief clerk through the door but has not tried to get out of his bed until now. After he falls from the bed, he tries to talk to his family and the chief clerk, but this time he makes mere bug noises:
"Quick, get the doctor. Did you hear the way Gregor spoke just now?"
"That was the voice of an animal", said the chief clerk, with a calmness that was in contrast with his mother's screams.
Immediately, the house is in an uproar as they try to find out how to get to Gregor. (And, they haven't even seen his bug-self yet.)
After great effort, Gregor manages to open the door. Upon seeing him, the chief clerk shouts out, "oh!" and his family responds with even greater shock:
Gregor's mother, her hair still dishevelled . . . unfolded her arms, took two steps forward towards Gregor and sank down onto the floor into her skirts . . . His father looked hostile, and clenched his fists as if wanting to knock Gregor back into his room
His mother goes into a state of immediate shock, falling down in weakness as she witnesses Gregor in his bug form. His father grows angry, with his fists clenched, as he aggressively urges Gregor back to the confines of his room. The chief clerk storms out of the building looking afraid, and Gregor retreats to his bedroom.
The only kindness Gregor receives after his transformation is from his sister. When he wakes up from his deep sleep he finds a plate of food:
By the door there was a dish filled with sweetened milk with little pieces of white bread floating in it. He was so pleased he almost laughed, as he was even hungrier than he had been that morning . . . . But he soon drew his head back again in disappointment . . . the milk did not taste at all nice.
Gregor leaves the food where it was placed; as a bug, Gregor is much more interested in rotten foods than fresh foods. Gregor's sister soon realizes this and begins to bring him the rotten, smelly foods that he desires. Immediately after his transformation, Grete attempts to show Gregor kindness.
However, as the novella continues, Grete grows less patient with Gregor and her kind acts change. First she neglects him; later, her growing anger and frustration becomes evident. She tells her father:
It's got to go . . . that's the only way, Father. You've got to get rid of the idea that that's Gregor. We've only harmed ourselves by believing it for so long.
Gregor, once the financial provider and leader of his household, is transformed into a helpless creature who depends on others for his survival. He loses his job and seemingly loses the affections of his mother and father upon his transformation into a bug. When he remains a bug, even his sister stops showing him kindness. Grete, meanwhile, goes through her own sort of metamorphosis from a carefree young lady with plenty of leisure time to practice her violin to a woman called upon to help provide for her family's finances.