Gregor Samsa wakes up to find that he has been transformed into a giant insect. Gregor briefly examines his new body, but wonders only momentarily about what has happened to him. His attention quickly switches to observing his room, which he finds very ordinary but a bit small, and a framed magazine clipping of a woman in fur hanging up on the wall. Since he can't turn on his side, Gregor cannot fall asleep, so instead he begins thinking about his job. He is a traveling salesman, and he hates traveling because he dislikes worrying and getting up early. Gregor's chief at work is extremely tyrannical, and Gregor wants to quit the job but cannot do so until he has paid off the debts that his parents owe the chief.
Gregor wants to get up to go to work, but suddenly realizes that he is already late and must have missed the alarm. He can't call in sick because he has not missed a day of work in five years and it would look suspicious. Gregor's mother calls to him, and he answers her, noticing that his voice is changing. Gregor's father and Grete, his sister, realize that he is still at home and try to enter his room, but he has locked his doors and they can't get in. Gregor attempts to get out of bed, but finds this very difficult. He realizes that he is now very late, and lies back hoping that some clear thinking will resolve the situation. Suddenly the doorbell rings and the chief clerk comes into the apartment. Angry that his firm sends the chief clerk himself if he is only a little late, Gregor finally swings himself out of bed.
As the family entreats Gregor to open the door, he refuses. Mrs. Samsa insists that Gregor must be ill or he would not be acting like this. The chief clerk loses his temper and tells Gregor that he is shocked by his attitude, insisting that his position in the company is not unassailable because his work has been poor lately. Gregor is angered by this speech, and insists that he is simply feeling slightly indisposed but will soon return to work. He retorts that his business has not been bad lately. Because of the changes in Gregor's voice, no one outside understands a word he says. Fearing he is ill, his parents send Grete and the servant girl to get the doctor and the locksmith. With great difficulty Gregor manages to open the door by himself.
Seeing Gregor, the chief clerk backs away while his father begins to weep. Gregor begs the chief clerk to explain the situation at the office and to stand up for him. He says that he will gladly come back to work and asks the chief clerk not to leave without agreeing with him. Gregor tries to stop the clerk so as to keep him from leaving with such a negative view of things, but then his mother, backing away, knocks over a coffee pot, causing a commotion and giving the chief clerk an opportunity to get away. Gregor's father picks up a walking stick to drive Gregor back into his room. Gregor gets stuck in the doorway, and his father shoves him through, injuring him in the process, and slams the door behind him.
Gregor wakes up at twilight and smells food. He realizes that his sister had brought him milk with bread in it. Gregor attempts to drink the milk, but finds that he is repulsed by the taste. Gregor notices that his father is not reading the paper to the family as he usually does and there is complete silence in the apartment. He wants someone to come in his room, but the doors are locked from the outside and no one will enter. Gregor climbs under the couch, where he feels more comfortable, and decides that he has to help his family through this difficult situation. Gregor's sister brings him a variety of foods in order to determine what he will eat. She throws away everything he doesn't finish, even if he hasn't touched it. Gregor hides under the couch to protect Grete from having to see him.
Assuming that Gregor can't understand anything, no one talks to him directly, so he learns what is happening by listening to their conversations through the door. He finds out that the family has money saved from his father's business, which had collapsed five year ago. Gregor had not known about this money, and when his father's business fell apart, he had thrown himself into his work in order to provide for his family. The family's initial excitement of receiving his earnings had worn off, however, and he remained intimate only with Grete, whom he had wanted to send to the Conservatory to study the violin.
Gregor watches his movements carefully, since any noise he makes distracts his family. He learns from their conversations that in addition to money from the business, the family has also saved money from his salary, but it isn't enough to live off of for very long. Gregor feels deep shame every time money is mentioned. He finds that his vision is getting worse, so that he can no longer see across the street. Every time Grete walks into the room, she runs to open the window, which bothers Gregor. Realizing that his sister is uncomfortable in his presence, Gregor figures out a way to cover himself with a sheet to keep out of sight. Gregor's parents never come into his room, and when his mother begs to see her son, the others hold her back.
Gregor discovers that he enjoys climbing the walls and the ceiling. Noticing this, his sister decides to give him more space by clearing the furniture from his room, and she asks her mother to help. Gregor's mother says that this will make it look like they are giving up on Gregor's recovery, but Grete disagrees. Hearing his mother's voice, Gregor realizes the importance of the furniture to him. The noise that the women make upsets him, and he decides to come out of hiding to save the framed picture on the wall from being taken. Seeing him, his mother faints and Grete runs out of the room for medicine to revive her with. Gregor follows and when his sister sees him she runs into his room and slams the door, trapping Gregor outside. His father arrives to find him out of his room and begins throwing apples at him. One of these lodges itself in Gregor's back, almost crippling him. As he loses consciousness, his mother begs her husband to spare her son's life.
Gregor's injury makes the family decide to be more accepting of him, and they leave his door open so he can watch them. They are very quiet most of the time and extremely tired from the jobs they have taken. No one bothers with Gregor too much. They have replaced the servant girl with a charwoman. Gregor, lying in his room, resorts to his memory. The family considers moving, but can't because they don't know how to move Gregor. He becomes angry that he is being neglected. Grete barely cleans his room and doesn't bother very much with his food anymore. When his mother tries to clean the room in Grete's absence, this triggers a family fight.
The charwoman, discovering Gregor, is not repulsed but rather spends her time teasing him, which annoys him to no end. Three lodgers have moved into the apartment, and the excess furniture, as well as all superfluous junk, is moved into Gregor's room so that he barely has room to move. He also stops eating almost entirely. The door to his room is now usually kept closed because of the lodgers, but Gregor doesn't care any more and often ignores it even when it's open.
The lodgers, who are domineering and receive too much service and respect from Gregor's parents, ask Grete to play the violin in the living room when they hear her practicing. She begins to play, but the lodgers are soon tired of this and move away to show that they are disappointed with her playing. Gregor, however, is drawn to the music and crawls out of his room to get closer, dreaming of getting Grete to play for him in his room and of telling her about his plans to send her to the Conservatory. The lodgers suddenly notice Gregor and give notice immediately, saying they will not pay for the time they have lived there.
Grete steps forward and tells her parents that they have to get rid of Gregor. He is persecuting them and trying to drive them out of the apartment and, if he really were Gregor, he would have left of his own accord and let them live their lives in peace. Suddenly realizing that he feels only love and tenderness for his family, Gregor understands that his sister is right and he should disappear. He returns to his room, waits until sunrise, and dies.
Gregor's family is happy, but they also mourn his passing. Mr. Samsa instantly kicks the lodgers out and the family decides to take the day off from work and go for a stroll. They feel relieved and the future seems bright to them. The parents notice that their daughter has grown up and decide that it is time to find her a husband. At the end of their trip, she is the first to stand up and stretch.