Entry Source: The James Bond Movie Encyclopedia by Steven Jay Rubin

SPECTRE’s scheme in Thunderball to steal two atomic bombs from a NATO bomber and then blackmail the North Atlantic Treaty powers to the tune of $280 million. The plan involves substituting an exact duplicate for NATO aerial observer François Derval (Paul Stassino), who will then hijack a bomber to the Bahamas with two atomic weapons on board. There, off Nassau, using a small army of underwater troops, SPECTRE agent number 2 Emilio Largo (Adolfo Celi) will hide the bombs until NATO pays the ransom. If something goes wrong, Largo has orders to destroy Miami with one of the bombs.