Thomas “Babe” Levy is a history Ph.D. student and avid runner researching the same field as his father, who committed suicide after being investigated during the Joseph McCarthy era. Babe’s brother, Henry, known as “Doc”, poses as an oil company executive but is actually a government agent working for a secret agency headed by Peter Janeway (William Devane). One of Doc’s jobs is to serve as a diamond courier for the infamous Nazi war criminal Dr. Christian Szell, in return for the latter’s assistance in tracking down other Nazi war criminals. Szell is known as der weiße Engel (German for The White Angel) due to his prominent mane of white hair. A wanted war criminal, Szell is ensconced in South America and is living off a large cache of diamonds which he had taken from Jews killed at Auschwitz. The diamonds are kept inside a safety deposit box at a bank in New York City and are withdrawn as needed by Dr. Szell’s brother. After Szell’s brother is killed in a traffic accident, Szell feels that he can’t trust anybody anymore and proceeds to have all of the diamond handlers and couriers murdered, including Doc himself.
Escaping several murder attempts, Doc suspects that Szell will come to New York to retrieve his valuable diamond collection. Doc comes to New York under the guise of a visit to Babe. Meanwhile, Babe and his new girlfriend, Elsa Opel, who claims to be from Switzerland, are mugged by two men dressed in suits. When Doc takes Babe and Elsa to lunch, he tricks Elsa into revealing that she has been lying to Babe about her background. Though Doc suspects she may be connected to Szell, he tells Babe that she is seeking an American husband so that she can become a U.S. citizen. After Szell arrives in America, Doc confronts him, stating that he shouldn’t have involved Babe in these matters and that he himself is not to be trusted. Szell then takes Doc by surprise and stabs him with a blade concealed in his sleeve. Doc makes it back to Babe’s apartment and dies.
The police interrogate Babe until government agents, led by Janeway, arrive. Janeway asks Babe what Doc told him before he died, and tells Babe that his brother was a U.S. government agent. Babe insists that his brother did not tell him anything, but Janeway is convinced Doc would not have struggled all the way to Babe’s apartment without giving him vital information.
Babe is later abducted from his apartment by the two men who mugged him in the park, and he is tortured by Szell. During his torture, Babe is repeatedly asked, “Is it safe?”, but he continues to deny any knowledge. Babe is then rescued by Janeway, who explains that Szell is in America to sell off his large cache of diamonds. Janeway presses Babe about Doc’s dying words, but Babe still insists he knows nothing. Frustrated, Janeway reveals himself as a double agent and returns Babe to Szell. Still unable to extract anything from Babe, Szell drills into one of his healthy teeth. Babe eventually escapes, aided by his skills as a marathon runner.
Babe phones Elsa, who agrees to meet him with a car. Arriving at her country home, Babe guesses that Elsa has set him up, forcing her to confess that the home was owned by Szell’s deceased brother. Janeway and Szell’s men arrive, but Babe takes Elsa hostage. Babe kills Szell’s men as they attempt to shoot him. Janeway offers to let Babe kill Szell in revenge for Doc’s death, if Janeway can have the diamonds. Babe agrees, but as he leaves, Janeway fatally shoots Elsa in revenge for her spoiling the plan to trap Babe and kill him in the house. Babe hears the gunfire outside the house and shoots Janeway through the window, killing him.
Attempting to determine the value of his diamonds, Szell visits an appraiser in the Diamond District in midtown Manhattan. A shop assistant who is a Holocaust survivor believes he recognizes Szell as a war criminal. After Szell hurriedly leaves the shop, an elderly Jewish woman also recognizes him. Trying to cross the street to get closer to Szell, the woman is hit by a taxi, causing a crowd to assemble to aid her. Amidst the confusion, the shop assistant appears again, directly confronting Szell, who slits the man’s throat.
Szell retrieves his diamonds but, as he attempts to leave, Babe forces him at gunpoint into Central Park. Babe tells Szell he can keep as many diamonds as he can swallow. Szell initially refuses, and Babe begins throwing the diamonds into the water. Szell relents and swallows one diamond, but then refuses to cooperate further. Babe throws the rest of the diamonds down the steps towards the water; Szell dives for them, but stumbles, and falls on his own knife blade, his dead body falling over a railing and into the water. Babe heads out into Central Park, stopping to throw his gun into the reservoir.