Insecure but entitled midwesterner Hart arrives at Harvard Law School. During the first day of class, legendary professor Kingsfield chides him for being unprepared. Unfortunately for the unsuspecting Kinsgfield, this casual snub inspires an imaginary rivalry in Hart's mind, one which carries us through Hart's first year at Harvard and deep inside the mind of an obsessed student.
Hart meets other students and forms a study group. They warn him of the stress of studying law under Kingsfield.
Walking home from the store one night, Hart meets a girl named Susan who lives near campus. When she goes inside, he lingers in the shadows, watching her through the windows. He feels a strange and inexplicable attraction to her. Susan's attention inspires him to speak aloud in Kingsfield's class, and this "success" arouses his libido. Viewing her as a weapon in his intellectual battle with the professor, he returns to her house, once again gazing uninvited into her windows before making his presence known. She invites him in, and he wakes her in the middle of the night to rave about Kingsfield and the competition in which they are engaged. She tries to comfort him.
His relationship with Susan continues until one morning he is late for class and unprepared. Kingsfield calls on him, and moves on to another student when Hart cannot answer the question. In Hart's mind, this routine practice is a grave insult and was directed at him personally because of their deep animus toward one another. Unable to psychologically cope with this perceived loss, Hart takes it out on Susan, blaming her for his failure and for not "giving him sustenance." He repeats this strange totemic phrase until he passes out drunk in the park for hours. When he wakes, Susan is gone. He curses her loudly and swears off women, who only distract from his struggle with the professor.
Returning home, he finds that Kingsfield has invited all of the students to a Thanksgiving party. At the party, he discovers that Susan is actually Kingsfield's daughter. He yells at her for not telling him of her connection to his enemy. With disgust, he tells her he doesn't even recognize her in dress-clothes. The viewer wonders why she was so secretive, but realizes that probably she did not tell Hart of her connection sooner out of fear that he might kill her and wear her skin like his own, so desperate is he to be closer to Kingsfield. But Susan is a woman with no personal will, so when Hart realizes that he now has the chance to become even closer to the professor, she takes up with him again. He grins lasciviously at her as he tells her she looks like her father. Both admit that Hart is only with her for this connection.
Susan resents her father deeply (her mother ended up in a mental institution) and makes love with Hart in her father's bed in order to spite him. Hart goes downstairs into the professor's office to gloat over his conquest, this victory over Kingsfield that Hart has won by defiling the man's daughter. He walks the edges of the office in his boxer shorts, swirling a snifter of brandy and basking in the glory of having bested such an important foe. But as Hart stands nearly nude in the library, Kingsfield returns home. Hart's cowardice causes him to flee without his clothes; this shame fuels his insecurity further.
He becomes paranoid, convinced that Kingsfield knows what he did. He tells Susan that Kingsfield is out to get him, that Kingsfield cares about him personally. He implies that Kingsfield watches him. He tells her he is inside Kingsfield's mind, that he is three steps ahead of Kingsfield's own thoughts. Susan tries to convince him that her father does not and cannot care who he is, but Hart cannot relinquish his delusions. Hart feels that every oppressive force on earth originates within the great professor. His relationship with Susan withers, as Hart is completely consumed by thoughts of her father. But Susan discovers Hart hiding in the shadows outside her house; he cannot let go of his only real connection with the professor.
He breaks into a library vault to view Kingsfield's notes from his time as a student, to reassure himself that Kingsfield is no better than him. He scoffs at them, but later, lying in bed, begins drawing symbols he saw doodled in the margin of Kingsfield's notes on the walls of his bedroom.
Hart begins reading every article or book Kingsfield ever published, trying to use them against Kingsfield in class discussions. Kingsfield mistakes this fixated combativeness for flattery and offers Hart a work assignment, but because of his bizarre obsession and eagerness to both please and defeat Kingsfield, Hart is unable to complete the task on time. Hart takes his failure so hard that he accosts Susan in a grocery store, blaming her and her father for his terrible insomnia and lack of appetite. Susan encourages him to stand up to her father, not understanding what this could mean for them all.
In class the next day, Hart once again fumbles when Kingsfield asks him a question. Kingsfield then tells Hart to call his parents to come pick him up, as he will never make it through law school. Hart retorts and calls Kingsfield a son-of-a-bitch, upon which Kingsfield says, "Mr. Hart, that is the most intelligent thing you have said all morning. You may take your seat".
Hart and a classmate check into a hotel. They are shown surrounded by trash, both in their underpants, muttering about Kingsfield's course. They return to school for exams. After the test, Kingsfield gets onto an elevator alone with Hart. In a pathetic turn, Hart attempts to express to Kingsfield how much he personally means to him, but Kingsfield does not even know Hart's name.
When Hart's grades arrive, he gleefully folds the envelope into an airplane and throws it into the ocean, seemingly uncaring of what the envelope signifies.