Kirylo Sidorovitch Razumov is a Russian student in St. Petersburg. He has never known his father and is supported by anonymous patron; he later learns this is his father, Prince K—. For most of his life, however, he has almost no family connections, and then his mother passes away. The narrator conjectures that Razumov needs to fill a void:

He was as lonely in the world as a man swimming in the deep sea. The word Razumov was the mere label of a solitary individuality. There were no Razumovs belonging to him anywhere. His closest parentage was defined in the statement that he was a Russian. Whatever good he expected from life would be given to or withheld from his hopes by that connexion alone.

One day, as Razumov is obsessing about studying for an important exam, a high-ranking government minister is killed by a bomb. It turns out that the killer is a fellow student, Victor Haldin, who shows up at Razumov's apartment seeking his help in escaping. While Haldin believes he is a committed revolutionary, he mouths slogans and rationalizes his violence and need to escape, mixing the benefit to the cause with his own self-importance. He tells Razumov,

Some day you shall help to build. You suppose that I am a terrorist, now—a destructor of what is. But consider that the true destroyers are they who destroy the spirit of progress and truth, not the avengers who merely kill the bodies of the persecutors of human dignity. Men like me are necessary to make room for self-contained, thinking men like you. Well, we have made the sacrifice of our lives, but all the same I want to escape if it can be done. It is not my life I want to save, but my power to do. I won't live idle. Oh no! Don't make any mistake, Razumov. Men like me are rare.

After much soul-searching, Razumov first decides to help Haldin but then realizes he would be jeopardizing his own life by doing so. He contacts Prince K—, who sets the process in motion that leads to Haldin's arrest and summary execution. Not only that, the state police appoint Razumov to the intelligence service and send him to Geneva—where there is a "Little Russia" community of revolutionaries and sympathizers—with orders to spy on them.

When Razumov arrives, he soon meets Haldin's sister, Natalia, of whom Haldin had spoken. Having expected her to be similar in politics and temperament, he is shocked to find her utterly enchanting. He suffers from a confusing mixture of guilt and resentment, as though it were her fault that he had betrayed her brother. Soon after, he realizes that he loves her:

The girl had existed for him ever since [Haldin had mentioned her]. But he did not recognize her at once. Coming up with Peter Ivanovitch, he did observe her; their eyes had met, even. He had responded, as no one could help responding, to the harmonious charm of her whole person, its strength, its grace, its tranquil frankness—and then he had turned his gaze away. He said to himself that all this was not for him; the beauty of women and the friendship of men were not for him. He accepted that feeling with a purposeful sternness, and tried to pass on.

When he summons up the courage to confess to her, he is overcome with shame and remorse. After admitting his role, he leaves her sitting there, stunned. From his room, he writes her a letter trying to explain his changing feelings for her, and the role he believes she was destined to play in his redemption:

Hate or no hate, I felt at once that, while shunning the sight of you, I could never succeed in driving away your image. I would say, addressing that dead man, "Is this the way you are going to haunt me?" It is only later on that I understood—only to-day, only a few hours ago. What could I have known of what was tearing me to pieces and dragging the secret for ever to my lips? You were appointed to undo the evil by making me betray myself back into truth and peace.

When news of his betrayal spreads, other members of the revolutionary cell avenge Haldin by attacking Razumov. After the vicious Nikita beats him, he also gets hit by a tram. Several others decide this punishment is too great, as he will never walk again. In particular, Tekla (who had been secretary to the cell’s leader) becomes disillusioned with the movement, cuts her ties, and goes back to Russia with Razumov. There he lives in her care, in

a little two-roomed wooden house, in the suburb of some very small town, hiding within the high plank-fence of a yard overgrown with nettles. He was crippled, ill, getting weaker every day, and Tekla the Samaritan tended him unweariedly with the pure joy of unselfish devotion. There was nothing in that task to become disillusioned about.