Gregor wakes up one morning to discover that he's become a "monstrous vermin" (1.1). As he struggles to come to terms with his new body, he realizes that he's late for his job as a traveling salesman. First his mother, then his father and sister, knock on his bedroom door in an effort to get him out of bed. His supervisor, the office manager, arrives to inquire about his absence. With his parents pleading with the office manager outside his bedroom door and his sister sobbing in another room, Gregor manages to crawl to his bedroom door, open it, and reveal to everyone his shocking new form. His mother collapses, and the office manager runs out of the apartment in horror. His father grabs a newspaper and the office manager's cane and chases Gregor around the living room. Gregor finally manages to crawl back to his bedroom door, but he gets stuck. His father firmly shoves him into the room and closes the door behind him.
Perplexed and horrified by Gregor's new body, both Gregor and the family settle into a routine in the following weeks and months. While Gregor gets to know the capabilities of his new body – and his new taste for rotten foods – Grete, his sister, becomes his primary caretaker, feeding him twice a day and cleaning his room.
One day, Grete discovers that Gregor enjoys crawling all around the room, including over the walls and the ceilings. Grete and the mother proceed to move the furniture out of Gregor's room to give him more space to roam. While up to this point Gregor has hidden himself whenever anyone walks into the room, he plants himself on top of a picture on the wall in an effort to express his wish that the furniture remains in his room. When the women return to the room, the mother sees Gregor and faints. Grete runs into the living room to get the mother some spirits, and Gregor follows. When Grete turns, she is startled by Gregor and runs back into Gregor's room. Flustered, Gregor scurries around the living room until he plops onto the table in the middle of the living room, exhausted.
After a brief while, the father returns home. Grete explains what has happened. The father, infuriated, chases Gregor around the living room and throws apples at him. One apple lodges into Gregor's back, paralyzing him. Suddenly, the mother runs from the room and begs the father to spare Gregor.
It takes a month for Gregor to heal from his wound. The door to Gregor's room is left open in the early evenings so that he can witness the family's nightly routine. While the father dozes in his bank messenger's uniform in a chair, the mother sews lingerie and Grete studies shorthand and French. The family hires a new cleaning woman, an old widow, who regularly chats with Gregor, much to Gregor's dismay. The family also takes in three boarders to make ends meet.
One night, the boarders invite Grete to play violin for them in the main room. Gregor is enthralled with Grete's playing, and creeps out into the middle of the room, in full view of all the spectators. At first amused, then horrified, the boarders declare that they intend to move out the next day without paying any rent. After the boarders retreat, the family confers. Grete insists that Gregor must be gotten rid of at all costs. Gregor, who is at this point still lying in the middle of the room, makes his way back into his room. Famished, exhausted, and depressed, Gregor dies early the next morning.
A few hours later, the cleaning woman discovers Gregor's corpse and announces his death to the family. After kicking out the boarders, the family decides to take a day off and take the trolley out into the country.