What is the main conflict in Thunderball by Ian Fleming?

James Bond, agent 007 with Her Majesty's Secret Service, is sent to a health clinic to recover from over-drinking and over-smoking. During his stay, he tangles with a criminal who, unbeknownst to Bond, is involved in an international blackmail scheme. Soon, Bond is whisked off to the Bahamas in search of SPECTRE, a criminal organization threatening to explode stolen nuclear bombs. Bond finally foils these modern-day pirates in a spectacular underwater battle.