Corporate engineer Joe Ross has developed a highly valuable "process," although its specifics remain a mystery. During a retreat on the picturesque island of St. Estèphe, he encounters the affluent stranger Julian "Jimmy" Dell—a meeting that piques the interest of one of the new secretaries at his company, Susan Ricci.
Jimmy introduces an enticing proposition: he wants Joe to meet his sister, an Olympic-class tennis player in New York, while requesting that Joe deliver a package to her. On their flight back to New York, Susan engages Joe in a conversation about the unpredictability of people, alluding to the concept of unwitting drug mules. Feeling uneasy that the package might contain something illicit, Joe opens it mid-flight, expecting the worst, but instead discovers a 1939 edition of Budge on Tennis, which he inadvertently damages in the process. Determined to make amends, he later acquires an intact copy and drops it off at Jimmy's sister’s building, retaining the damaged original at his office.
As their friendship develops, Jimmy insinuates that Joe's boss, Mr. Klein, may not fully compensate him for his innovation. He dismissively opens a Swiss bank account for Joe with a token amount of 15 Swiss francs and invites him to a membership-required dinner at an exclusive club, where he encourages Joe to seek legal advice regarding his work’s value. This friendly gesture, however, masks Jimmy's ulterior motives. Upon further investigation, Joe uncovers the shocking truth: Jimmy's sister doesn't exist, and his new friend is a con artist intent on swindling him out of the "process."
Desperate and confused, Joe reaches out to Pat McCune, a woman he met on the island whom Susan claimed was an FBI agent. Intrigued by Joe's situation, McCune's unit enlists him for a sting operation to apprehend Jimmy. As agents prepare Joe with a wire to monitor their meeting, he learns about the Spanish Prisoner con—an elaborate deceit Jimmy has ensnared him in. However, when Jimmy fails to meet, it quickly dawns on Joe that McCune may be part of the con as well, and the "process" has vanished into thin air.
When Joe attempts to explain the situation to his employer and local authorities, he finds himself in a nightmarish maze of deception: Jimmy has made it seem as though Joe sold the process to a Japanese firm. The Swiss bank account marks him as a suspect hiding wealth, while the membership certificate unknowingly signed by Joe was a covert request for political asylum in Venezuela—a nation without an extradition treaty with the U.S. The police reveal that Jimmy's chic apartment is merely a front and that the exclusive members-only club is nothing more than a mundane restaurant. To compound matters, Joe is framed for the murder of the company attorney, George Lang.
On the run, Joe finds solace in reconnecting with Susan, who expresses her belief in his innocence, providing a glimmer of hope. He recalls that the hotel on the island has video footage that could validate his claims about Jimmy's presence there. Subsequently, Susan takes Joe to the airport so he can travel back to the island. However, upon spotting a police checkpoint en route to the airport, she cleverly persuades him to reroute to Boston.
At the Boston airport, Susan gifts Joe a plane ticket and a camera bag that intriguingly contains a firearm—information Joe is blissfully unaware of. Just as Joe starts to piece things together, realizing that Jimmy's fingerprints are on the book he was to deliver, he leaves the airport with Susan. In a twist of fate, while attempting to board a ferry home, he discovers that the ticket is actually for Venezuela, shattering his sense of security and signifying a setup.
The story reaches its climax on the ferry when Jimmy suddenly confronts Joe, and in a shocking turn, Susan betrays Joe, revealing that the final act of the con will lead to Joe’s demise, staged as a suicide. Jimmy, now cornered, divulges his manipulations concerning the process. However, before he can execute his plan, he's subdued by U.S. Marshals disguised as Japanese tourists. The agents reveal they've been monitoring Jimmy's activities for months, all while Mr. Klein orchestrated the entire ruse to claim the profits for himself. In a chilling moment, Susan pleads for Joe's mercy, to which he coolly responds that she must "spend some time in [her] room," implying a prison sentence for her role in the betrayal.
