It takes a special type of person to serve in a nation's navy, especially on long voyages that separate men and women from their loved ones, and no service is both loved and hated as that aboard submarines, for very few people ever serve on them on a whim. For one thing, the psychological impact of being trapped for long periods underwater in tight, cramped quarters is more than many people can stand. Also, submarine service is uncharacteristically hazardous; after all, if a surface vessel is sunk, the crew has a reasonable chance of escaping death in lifeboats or being rescued out of the water by another ship. Conversely, if a submarine is badly damaged while submerged, the crew's chances of survival are at best remote.

Given that there's such little margin for error in a submersible, many submarine losses remain sources of intrigue and mystery, and few rival the disappearance of the USS Thresher in that regard. In 1963, the Cold War between the Soviet Union and the United States was at its peak, as the Cuban Missile Crisis the previous year had brought the world to the brink of nuclear war and tensions were still running high. In space, the Soviet Union seemed to have taken the lead, putting the first satellite in Earth's orbit and placing the first man in space.

There was also another new theater of war, but one that was little-known at the time. Under the waters of the world's oceans, Soviet and American nuclear submarines were in intense competition, pushing the boundaries of new technologies. Some of these submarines carried nuclear missiles that carried more destructive power than all the bombs dropped by the U.S. Army Air Force throughout World War II, including the nuclear bombs dropped on Hiroshima and Nagasaki. Other submarines, the hunter-killers, were designed to find and destroy these missile-carrying submarines. All of these submarines engaged in their activities while aiming to avoid detection by the other side, establishing a clandestine conflict that was carried out far from the gaze of the public but still strategically vital. If either side could gain a notable advantage, it could abruptly change the fragile balance of power.

Then, in April 1963, one of the most advanced U.S. hunter-killer nuclear submarines and the 129 men onboard vanished while on a routine voyage. The USS Thresher was one of the country's most advanced ships, but it had the misfortune of becoming the first American nuclear submarine to be lost at sea, prompting obvious questions over what had happened. Had it accidentally collided with a Soviet ship? Was it destroyed by a new and secret Soviet weapon? Was it secretly captured and taken back to the Soviet Union to reveal its secrets?

As it turned out, it would take longer than anyone could have guessed to find out what actually happened.