One day, Gregor Samsa, a traveling salesman, wakes up to find himself transformed into a giant insect (the most common translation of the German description ungeheures Ungeziefer, literally "monstrous vermin"). He believes it is a dream and reflects on how dreary life as a traveling salesman is. As he looks at the wall clock, he notices that he has overslept and missed his train for work. He ponders the consequences of this delay. Gregor becomes annoyed at how his boss never accepts excuses or explanations from any of his employees no matter how hard-working they are, displaying an apparent lack of trusting abilities. Gregor's mother knocks on the door, and he answers her. She is concerned for Gregor because he is late for work, which is unorthodox for him. Gregor answers his mother and realizes that his voice has changed, but his answer is short, so his mother does not notice. His sister, Greta, to whom he was very close, then whispers through the door and begs him to open it. He tries to get out of bed but is incapable of moving his body. While trying to move, he finds that his office manager, the chief clerk, has shown up to check on him. He finally rocks his body to the floor and calls out that he will open the door shortly.
Offended by Gregor's delayed response in opening the door, the clerk warns him of the consequences of missing work. He adds that Gregor's recent performance has been unsatisfactory. Gregor disagrees and tells him that he will open the door shortly. Nobody on the other side of the door could understand a single word he uttered (Gregor is unaware that his voice has also transformed), and they conclude that he is seriously ill. Finally, Gregor manages to unlock and open the door with his mouth. He apologizes to the office manager for the delay. Horrified by Gregor's appearance, his mother faints, and the manager bolts out of the apartment. Gregor tries to catch up with him, but his father drives him back into the bedroom with a cane and a rolled newspaper. Gregor injures himself squeezing back through the doorway, and his father slams the door shut. Gregor, exhausted, falls asleep.
Gregor awakens and sees that someone has put milk and bread in his room. Initially excited, he quickly discovers that he has no taste for milk, once one of his favorites. He settles himself under a couch. The next morning, his sister comes in, sees that he has not touched the milk, and replaces it with rotting food scraps, which Gregor happily eats. This begins a routine in which his sister feeds him and cleans up while he hides under the couch, afraid that his appearance will frighten her. Gregor spends his time listening through the wall to his family members talking. They often discuss the difficult financial situation they find themselves in now that Gregor can't provide for them. Gregor had plans of sending Greta to the conservatory to pursue violin lessons, something everyone else – including Greta – considered a dream. His incapability of providing for his family, coupled with his speechlessness, reduces his thought process greatly. Gregor also learns that his mother wants to visit him, but his sister and father will not let her.
Gregor grows more comfortable with his changed body. He begins climbing the walls and ceiling for amusement. Discovering Gregor's new pastime, Greta decides to remove some of the furniture to give Gregor more space. She and her mother begin taking furniture away, but Gregor finds their actions deeply distressing. He tries to save a picture on the wall of a woman wearing a fur hat, fur scarf, and fur muff. Gregor's mother sees him hanging on the wall and passes out. Greta calls out to Gregor—the first time anyone has spoken directly to him since his transformation. Gregor runs out of the room and into the kitchen. He encounters his father, who has just returned home from work. The father throws apples at Gregor, and one of them sinks into a sensitive spot in his back and remains lodged there, paralyzing his movements for a month and damaging him permanently. Gregor manages to get back into his bedroom but is severely injured.
One evening, the cleaning lady leaves Gregor's door open while three boarders, whom the family has taken on for additional income, lounge about the living room. Greta has been asked to play the violin for them, and Gregor – who usually took care to avoid crossing paths with anyone in the flat – in the midst of his depression and resultant detachment, creeps out of his bedroom to listen. The boarders, who initially seemed interested in Greta, grow bored with her performance, but Gregor is transfixed by it. One of the boarders spots Gregor, and the rest become alarmed. Gregor's father tries to shove the boarders back into their rooms, but the three men protest and announce that they will move out immediately without paying rent because of the disgusting conditions in the apartment.
Greta, who has by now become tired of taking care of Gregor and is realizing the burden his existence puts on each one in the family, tells her parents they must get rid of Gregor, or they will all be ruined. Her father agrees, wishing Gregor could understand them and would leave of his own accord. Gregor does, in fact, understand and slowly moves back to the bedroom. There, determined to rid his family of his presence, Gregor dies.
Upon discovering Gregor is dead, the family feels a great sense of relief. The father kicks out the boarders and decides to fire the cleaning lady, who has disposed of Gregor's body. The family takes a trolley ride out to the countryside, during which they consider their finances. They decide to move to a smaller apartment to further save money, an act they were unable to carry out in Gregor's presence. During this short trip, Mr. and Mrs. Samsa realize that, in spite of going through hardships which have brought an amount of paleness to her face, Greta appears to have grown up into a pretty and well-figured lady, which leads her parents to think about finding her a husband.