Plot

A former U.S.Justice Department official is hired as a security consultant for a Zurich based Swiss bank when five of its clients are blackmailed.

When a Swiss bank finds that the confidentiality of some of its more vulnerable customers has been compromised it calls in an American investigator, who soon uncovers a web of deceit and blackmail. With old debts being paid off his own health is soon in danger, but at least he starts to gets to know one of the bank's female customers pretty well.—Jeremy Perkins {J-26}

The banking industry in Switzerland is famous for maintaining secret numbered accounts, offering an ideal place to hide illicit funds of international criminals due to the privacy laws of the Swiss banks. Recently, however, Hurtil Bank, in Zurich, Switzerland, has received anonymous letters threatening to expose the names of depositors unless the bank pays ten million Swiss francs. Additionally, blackmail letters, asking for one million francs each, have been sent to five Hurtil account holders: Denise Abbott (Senta Burger), an attractive European socialite; Robert Hayes (John Saxon), a gangster from Chicago, Illinois; Dwight McGowan (John Ireland), a rich, corrupt businessman from Texas; Mr. Kosta (Curt Lowens) a businessman based in Amsterdam, Holland; and Mr. Rascha, an arms dealer, who was mysteriously murdered in a restaurant after receiving his letter.

In an effort to avoid scandal and police involvement, bank president, Johann Hurtil (Ray Milland), hires American David Christopher (David Janssen) to investigate. Christopher, a former U.S. Justice Department official, runs an international consulting firm based in Geneva, Switzerland. During his initial meeting with Hurtil, Christopher is introduced to Denise, who mentions that the criminals must know she does not have a million francs, if they have access to her account.

While at the bank, Christopher also encounters Hayes, who recently arrived in Zurich to confront Hurtil and vice-president Franz Benninger (Anton Diffring) about the blackmail. Upon learning Christopher has been entrusted with the case, Hayes is furious since Christopher was responsible for prosecuting him and his mafia associates in the U.S. Later, Christopher is en route to meet Denise at her hotel when Hayes tries to shoot him, but Christopher defends himself and exchanges blows with the gangster. Hayes flees, just before Captain Heinz Frey of the Swiss police arrives on the scene.

As manager of Zurich's Federal Bank Detail, Frey is curious about Christopher's business with Hurtil, but the consultant remains guarded about the investigation. During his appointment with Denise that evening, Christopher learns that she had a love affair with Lord James Ashwood, a prominent British politician. If her Swiss account is compromised, Denise fears the romance will be exposed, causing a scandal for Ashwood and his family.

Meanwhile, Robert Hayes is murdered by two assassins, and Frey and Christopher discover the body later that night. The following day Christopher's assistant, Connie, provides background profiles on the five account holders and reports that Hurtil and Benninger appear to be clear of any wrongdoing. Next, Christopher interviews McGowan, who states that he was careful not to share his Swiss account with anyone and cannot afford another setback since the U.S. Internal Revenue Service is investigating him for tax evasion.

Back at the bank, Hurtil reads Christopher the final instructions from the blackmailers. Christopher and the three remaining blackmailed clients must deliver uncut diamonds worth 14 million francs to a location in Interlaken, Switzerland. Since the situation is more dangerous in the wake of two murders, Hurtil agrees to pay the entire ten million demanded from the bank, and the one million demanded from each of the five account holders.

During the meeting, Kosta is present, and Christopher asks the Dutchman why a blackmailer would be interested in him, but Kosta does not have an answer. Later, Christopher succumbs to Denise's charm and spends the night with her. As the investigation continues, he suspects Franz Benninger and his girl friend, Rita Jensen, might be the blackmailers, after Christopher notices the vice-president furtively handing a portfolio to Jensen outside the bank. However, Christopher and Hurtil soon learn Benninger is innocent and can only be blamed for surreptitiously helping Rita access her deceased father's account for which she did not have documentation. With Benninger absolved, Christopher pursues another lead and informs Hurtil that he is flying out of town, but will return tomorrow, in time to comply with the blackmailer's instructions.

At the airport, Christopher discloses details about the investigation to Frey, and the policeman grants him 24 hours to settle the case before he alerts Swiss authorities. Arriving back in Zurich the following day, Christopher is given a bag of uncut diamonds from Hurtil, then travels to Interlaken, accompanied by Kosta, Denise, and McGowan. There, the group receives further instructions from the blackmailer to board a steam train, but McGowan says he feels unwell and must return to his hotel. After the train ride up the mountain, Christopher, Kosta, and Denise are told by the conductor to proceed to the ski lift platform where an anonymous telephone caller orders Christopher to take the diamonds to the peak while Kosta and Denise remain below, in plain sight. However, Kosta offers to deliver the ransom instead, stating that he is more valuable to the blackmailers and, therefore, less likely to be killed. Christopher consents, and Kosta departs with the diamonds. Suddenly, Christopher changes his mind and follows Kosta, while pulling Denise onto the chair lift with him.

Meanwhile, Frey and his men are en route to Interlaken to provide support for Christopher. During the ride on the ski lift, Christopher confronts Denise, revealing that he flew to London, England, yesterday to interview Lord Ashwood, who disclosed that she has been blackmailing him ever since the politician ended their affair. Christopher declares that Denise's talent as a mistress of wealthy men is the link between Kosta, McGowan, Rascha, and Hayes, and, together, the five of them organized the blackmail. He also blames her for the deaths of Hayes and Rascha.

Elsewhere, McGowan does not return to Zurich and instead takes a helicopter to the top of the ski lift where he waits for the arrival of the diamonds. As soon as Kosta reaches the top, McGowan betrays his accomplice, shooting the Dutchman dead. McGowan tries to escape with the diamonds in the helicopter, but Christopher disables the tail rotor with a gunshot, then kills McGowan. After Christopher retrieves the diamonds from the snow, Denise attempts to charm him and acquire the precious stones, but he is no longer deceived by her cunning. When Denise aims a gun at him, Christopher outwits her, just as Frey arrives to apprehend the socialite.