Thomas "Babe" Levy (Dustin Hoffman) is a history Ph.D. candidate and avid runner. Babe is researching the same field as his father, who committed suicide after the Communist witch hunts of the Joseph McCarthy era ruined his reputation. Babe's brother Henry (Roy Scheider), known as "Doc", presents himself as an oil company executive but is really a government agent, involved in an elaborate network of couriers who transport diamonds stolen during World War II from wealthy Jews seeking to flee Germany. The diamonds are sold for the benefit of fugitive Nazi war criminal Dr. Christian Szell (Laurence Olivier), who in return helps in the hunt for other Nazi war criminals. Szell, a dentist who tortured Jews in a concentration camp, escaped capture and is now living off the diamond sales as a fugitive in South America (echoing real life Nazi war criminal Josef Mengele).
Christian Szell's brother, 72-year-old Klaus (Ben Dova), lives outside New York City and serves as the initial diamond courier, but is killed in a Manhattan road rage incident. Although Klaus' death was accidental, Christian suspects foul play and immediately puts everyone in the courier network under suspicion (anyone who obtains one of the two keys to the safe deposit box would immediately have access to a fortune in diamonds). Deducing this from hearing about the incident from his boss Peter Janeway (William Devane), Doc realizes that Christian Szell will have to travel to New York to retrieve the diamonds.
After discovering the disappearances or deaths of fellow couriers and escaping an two attempts on his own life in Paris, Doc goes to New York under the guise of a visit to Babe. Meanwhile, Babe and his new girlfriend, Elsa Opel (Marthe Keller), who claims to be from Switzerland, are mugged by two men dressed in suits (whom we later learn are Nazi fugitives connected to Dr. Szell). Babe assumes the attack to be a random mugging, but Doc knows it's a warning for him.
When Doc takes Babe and Elsa to a French restaurant, he suspects Elsa is connected to Szell, and tricks Elsa into revealing that she has been lying about her background, and warns Babe against her by claiming that she doesn't care for him and is only seeking an American husband so that she can become a U.S. citizen. After Szell arrives in America (and is met at the airport by the two men who mugged Babe and Elsa), Doc angrily confronts the three, accusing Szell of crossing a line by involving his brother. Szell, after questioning Doc as to his own safety, then stabs Doc with a blade concealed in his sleeve. Doc makes it back to Babe's apartment before dying in his arms.
The police interrogate Babe then Doc's boss, Janeway, arrives and questions Babe on what Doc told him before he died. Janeway reveals that Doc was a government agent, working for a secret branch known as "The Division." 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 revealing something, and sets Babe as bait.
Babe is abducted from his apartment by the muggers, and Szell tortures him using a dental probe on a cavity in his tooth and repeatedly asking "Is it safe?" (to retrieve his diamonds). Babe truthfully denies any knowledge, but Szell tortures Babe relentlessly regardless of his answers. Babe is rescued by Janeway, who explains that Szell is in America to recover the 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. Making a final attempt to extract information from Babe, Szell drills into one of his healthy teeth, but gets nothing so Szell orders his men to "get rid of him". Babe escapes as they get him into their car, and aided by his skills as a marathon runner, evades Janeway's attempt to recapture him.
He gets to his neighborhood and sees that Janeway and Szell's men are now waiting outside his apartment building, Babe arranges with hoods in the neighborhood, to sneakily procure a gun from his apartment. He then calls Elsa from a pay phone, and while ostensibly driving him to a safe place, she arrives at a country home, to Babe's increasing suspicion. Babe concludes that Elsa is also part of Szell's network and has set him up, forcing her to confess that the home was owned by Szell's deceased brother.
Janeway and Szell's men arrive at the house and Babe takes Elsa hostage with his gun. As one of Szell's men reaches for his gun, Babe shoots him; Janeway joins in and shoots both of Szell's men, saying that they couldn't be trusted. Janeway then reveals where Babe can find Szell, ostensibly as restitution to Babe for Szell's murder of Doc, and Elsa implores Babe to leave. As Babe turns to leave, however, Janeway prepares to shoot Babe in the back. Elsa sees this and yells out a warning to Babe; so Janeway shoots Elsa and Babe manages to kill Janeway, then leaves to find Szell.
While attempting to determine the value of his diamonds, Szell is recognized by the shop assistant, who is also a Holocaust survivor. 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. Amid the confusion, the shop assistant follows and confronts Szell, who slits the man's throat.
Szell retrieves his diamonds from the bank, but as he is leaving, Babe appears behind him and tells him, "It's not safe" and forces him at gunpoint to walk to a water treatment plant in Central Park (where Babe regularly runs). Szell, in an attempt to negotiate his escape, offers Babe a portion of the diamonds; Babe declines, telling Szell he can keep as many diamonds as he can swallow. Szell initially refuses, so Babe taunts Szell taking his briefcase and throwing handfuls of diamonds at him, which fall through the grating platform they're standing on and into the water below. Szell, aghast at Babe's actions, relents and swallows one diamond, but then refuses to cooperate further.
Szell tries to get close to Babe to stab him, accusing his late father and brother of being as weak and predictable as Babe is, and spits at him; Babe strikes Szell, but loses his grip on his gun. Szell then reveals his dagger and lunges at him, but Babe manages to avoid it and throw the open briefcase with the remaining diamonds down a stairwell towards the water. Szell dives towards the diamonds, but stumbles and rolls down the steps, fatally falling on his own knife blade. Babe heads out into Central Park, throwing his father's gun into the reservoir, and symbolically walks away in the opposite direction of the park's running patrons.
Who tortures Babe?
Szell