Today, two weeks after Mr. Kerviel's beach break was cut short, a picture is starting to come together of the junior trader who, undetected, caused the largest single trading hit in banking history. Previously unreleased details of the transaction that first brought him down, along with interviews with bank executives, prosecutors and Mr. Kerviel's own lawyers, reveal a man who knew the risks but thought he could beat them -- and whose undoing has exposed the frictions within one of France's proudest and most elite banks.