U.S. government agent Harry Hannan suffers a nervous breakdown after his wife Dorothy is killed, and spends five months in a sanitarium. Upon returning to his apartment in New York City, he finds it is occupied by a doctoral student named Ellie Fabian. She explains that she had a sublet arranged while she was in the last semester of her studies at Princeton University. Ellie claims that the housing office said the Hannans would be gone indefinitely. She gives Harry a note that was slipped under the door, but it contains only a few Hebrew characters that he cannot read.
Paranoid that he is being targeted by his own agency, Harry visits his supervisor Eckart, who assures Harry that the agency has higher priorities. Eckart insists that Harry is not ready to return to the field, but that he is perfectly safe.
Harry takes the Hebrew note to a local rabbi who can only partially decode it, and explains that it means "Avenger of Blood.". The rabbi then calls Sam Urdell, and informs him that Harry has visited him. Harry notices that he is being surveilled, loses the tail and goes to the American Museum of Natural History, where Ellie is working.
He gives her some money and urges her to stay in a hotel, because he fears she will be accidentally targeted. He then visits his wife's grave, where he confronts her brother, Dave Quittle. Afterwards, Quittle visits Eckhart, who orders Harry's murder.
Ellie stays in the apartment despite Harry's request. Ellie suggests that they take the note to her friend at Princeton who specializes in Hebrew studies. When Harry wakes from a nightmare, he tells Ellie about the death of Dorothy. He takes a prescription pill, but spits it out, realizing that it is cyanide. The next morning, they leave for Princeton. On the train, Ellie tells Harry about her grandmother, when Harry notices Quittle, and an old man, watching them.