One morning upon waking from an anxious dream Gregor Samsa finds himself turned into a giant venomous insect. As he lay on his back, his thin little legs struggling in the way up turned bugs do he wonders what has happened. He glances around his room, and find that everything as he had left it. His cloth samples were spread around the room, the picture that he had only recently cut out of a magazine and framed was there. But Gregor Samsan has unquestionably become a bug. He attempts to roll over and go back to sleep, but his insect body prevents them. He begins to feel sore and strained from the effort and stops.

Gregor lies in bed thinking about his demanding job as a traveling salesmen. He hates everything about it, waking up, the bad food, and the temporary nature of all his relationships. He thinks that in five or six years when he has paid off his parents debts he will be able to quit. In the midst of his musings Gregor looks over at the clock and realizes to his alarm that he had slept through his alarm and was now quite late for work. He realizes that his boss is going to be infuriated. Gregor ran through possible explanations in his mind, could he report in sick? No, he hadn't done that in all the five years he had worked there. He determined that his boss would bring the doctor who would simply proclaim that he is too lazy to work.

A knock at the door interrupts his thoughts. It is his mother informing them that he is very late, and asking him if he wants to get up. Gregor responds, but when he does he notices that his normal voice is now effected with a squeaky quality. When his father and sister Grete realize that he is still at home they also come to his door and plead for him to get up. Gregor's father points out that his recent performance at work has not been good, and that showing up late for work might exacerbate rumors that he has recently stolen money from his job. At their urging Gregor twists about wildly in bed and manages to free himself. Just as he is able to get up, the doorbell rings. It's the office manager checking to see why Gregor didn't make it to work, and asking if he can go into Gregor's room to speak with him personally. Gregor assures him that he is coming to work "right away," but the manager excoriates him for being irresponsible, and demands an explanation. Gregor assures him that he had just been suffering a slight dizzy spell, and will be ready for work post haste. As the manager becomes more and more aggravated, his family in turn becomes increasingly concerned. Everyone is concerned about the animalistic quality to Gregor's voice. They beginning thinking that they should fetch both a doctor, and a locksmith.

Meanwhile, Gregor himself is slightly confused by the dramatics of the morning. Finally, he is able to open his door by turning the key with his mouth and forcing his body against it. When everyone sees that Gregor is now a giant insect, they have varying reactions of shock. His mother faints, and the office manager backs against the wall. Gregor, however, calmly explains that he will now pack up his cloth samples and be on his way. He asks the manager to put in a good word for him with the boss. He also begs the man not to give his parents and sister any further trouble on his behalf. The manager, terrified by the situation, attempts to flee the apartment, and Gregor pursues him. His father, however, takes up a cane and a newspaper, and chases him back to his room where he slams and locks the door. Gregor is injured in the tableau and, bleeding, he falls asleep.

When he wakes up it is dark, and he finds that someone has left a bowl of bread and milk on his floor. Delighted, Gregor puts immerses his entire head in his favorite substance, but is disappointed to find that the taste no longer appeals to him. Gregor listens for the familiar sounds of his family, but the house is completely abandoned. He recalls his time spent providing for them with pride, and wonders what will become of them now that he is an insect. Gregor spends the evening crawling around the room. At one point someone cracks the door open, but quickly shuts it. Exhausted and worried, Gregor crawls under a sofa and falls asleep determined to make sure his new condition is not a burden on his family.

In the morning Grete opens the door to Gregor's bedroom, and to her horror finds him sleeping under the sofa; still a giant insect. Startled, she closes the door, but opens it again because she feels bad. When she notices that he hasn't eaten she brings scraps from the kitchen. Gregor is able to eat the moldiest food, but find the fresh vegetables unappealing. This marks the beginning of a pattern in which Grete feeds Gregor, and reports back to their parents how much he is consuming.

Gregor spends much of his time at his door listening to his family converse. He learns that they have saved a bit of the money that he has provided for them, and are not in any immediate financial danger. However, it's clear that some family members will have to get jobs if they want to avoid economic hardship, which makes Gregor feel ashamed.

Gradually, Gregor adapts to his new life. Grete continues to take care of him, but can hardly stand looking at him and encourages her parents to stay away from them to avoid the trauma. She notices that he seems to take pleasure in crawling around the room, and so she decides to take out all of his furniture and possessions to make more room for him. Gregor is extremely upset by the idea of losing all vestiges of his humanity, and attempts to prevent her from removing his favorite picture by crawling up the wall and hiding it with his body. When Gregor's mother walks in and finally sees him on the wall she panics and faints. Grete shouts at her brother, and panicked, he flees into the living room. When his father returns home, he misreads the situation, and thinking that Gregor has escaped and intentionally attacked his mother, he chases his son around the house throwing things at him as his wife pleads for him to stop. Finally, Gregor's father hits his son with an apple that becomes logged in his back.