The alleged plot reads like a film treatment for a spy thriller in the secretive and moneyed world of international private banking. The setting is Santiago, Chile.

A surveillance camera catches an executive from one international bank taking several boxes and bags of documents through the back door of a competitor's office. The mole is working in cahoots with two employees of the rival bank, who pass him off to other employees as a compliance officer from headquarters.

The scene is a slice of a pending lawsuit in federal court in Miami. In it, American Express Bank alleges that Credit Suisse conspired to poach rich clients with total assets of $127 million from A.E.B.'s Santiago office.

Further, American Express contends that Credit Suisse pulled off the bank job with the help of two A.E.B. private banking managers, Christian Echeverria and Jorge Salame.

The Credit Suisse operative named in the lawsuit, David Rosenkranz, is a Swiss citizen, born in 1960. The complaint, filed May 13, gives the number of his passport: F xxx2314.

Court records show that American Express accuses Rosenkranz of having posed as an A.E.B. compliance officer during a 10-day visit to Santiago in December 2007, working in a small conference room where he used flash drives and one of A.E.B.'s computer printers.

Fights over clients are not uncommon, said Christopher Stief, who runs the employee defection and trade secrets practice at the law firm Fisher & Phillips in Radnor, Pennsylvania. But they usually erupt when an employee jumps ship and tries to bring clients along.

"This is more like industrial espionage," Stief says. "There are two highly sophisticated players on each side. This is a highly unusual scene."

The prologue for this bit of corporate espionage began in February 2007, when American Express began negotiations to sell the A.E.B. to Standard Chartered, the London bank. They reached an agreement on September 16, 2007.

In the interim, though, Echeverria and Salame agreed to set up their own business and work for Credit Suisse as "external business managers." That, at least, was a plan laid out in a July 20, 2007, PowerPoint presentation that A.E.B. later discovered on its computers. The A.E.B. computers also contained July 20 letters in which Credit Suisse offered to hire Echeverria and Salame.

By December, when Rosenkranz visited Santiago, the plot thickened, according to the complaint: Not only did the Credit Suisse banker use a pen drive to copy A.E.B. documents, he also began telling A.E.B. clients that Standard Chartered "did not do private banking; was not interested in Latin America."

This falsehood, which was also disseminated by some A.E.B. employees working "in concert" with him, led some wealthy clients to conclude that "there would no longer be private-banking capabilities for A.E.B. clients in Chile," the complaint says.

A.E.B. clients were advised to close their accounts and transfer their considerable assets to Credit Suisse, and that—this is the delicious part—"they could come in to the A.E.B. office to sign papers to do so," documents say.

Apparently, nearly a quarter of A.E.B.'s Chilean business—more than 80 clients—bought this line, representing $127 million in assets as of May 9, 2008.

But there is no perfect crime, and, at least according to the A.E.B. complaint, there was a fatal flaw in this scheme: Lunch. Yes, lunch! While most of A.E.B.'s Chilean staff joined the renegades in their deed, according to the complaint, two A.E.B. employees photocopied Rosencranz's papers while he was at lunch.

"Many of these papers contained the Credit Suisse logo," according to the complaint.

The sale of A.E.B. to Standard Chartered closed on February 29, 2008. At that time, Echeverria traveled to A.E.B.'s Miami offices to talk about the deal, according to the complaint, which characterizes him as having been "evasive" at a meeting.

On April 3, A.E.B. fired Echeverria, as well as Salame and most of the rest of its employees who had joined the plot to steal clients. A day later, A.E.B. began to receive orders from clients, faxed from the new company and from some clients themselves, requesting the transfer of accounts to other institutions, primarily Credit Suisse.

Stief, for one, is not convinced that Credit Suisse managers were on board with the scheme. If Credit Suisse could establish Rosenkranz as a "rogue employee," that would go a long way toward mitigating the bank's main worry: the fear of punitive damages.

For Stief, this lawsuit triggers all sorts of wildness, not the least of which is that the Gramm-Leach-Bliley Act of 1999 requires financial institutions to protect customer data. "They are all sensitized to the fact that sending a guy into a competitors' office is a radical move," he says.

For now, A.E.B. has convinced U.S. District Judge William Hoeveler of its case: He granted the bank's motion for a temporary restraining order, finding that it "clearly appears" that Credit Suisse "has stolen A.E.B.'s trade secrets and other confidential and proprietary information" and "is making false and misleading statements about A.E.B."

The restraining order was granted May 14, and shall last 10 days. A.E.B. is seeking to extend the restraining order for another 10 days through the Memorial Day holiday.

A.E.B. is represented by James Beasley Jr. of Beasley Hauser Kramer Leonard & Galardi. David Walker, a Credit Suisse spokesman in New York, declined to comment on the lawsuit.