The story of the underwater war between US, UK and Soviet submarines in the second half of the 20th century.
　　Part 1: Know Your Enemy
　　For decades, Britain and America's Cold War submarines waged a secret war of espionage against the Soviet navy. Deep in the ocean, crews were locked in a game of cat and mouse as each side battled to gain the tactical and technological advantage. After decades of silence, submariners from both the east and west are now allowed to talk more openly than ever before about how they plotted to win the war beneath the waves. The west's superior technology allowed them to secretly shadow the Soviet fleet, at close quarters, giving them vital intelligence and the upper hand if war broke out. Shadowing submarines was dangerous. The film explores close encounters between western and Soviet forces that put the lives of submariners at risk. Candid interviews with British, American and Russian submariners reveal the pressures of lengthy underwater patrols that drove them to the edge of their physical and mental limits.
　　Part 2: The Russians are Coming
　　Throughout the 1970s and 80s, the front line of the Cold War was hidden beneath the ocean. Submariners from three navies - American, Soviet and British - played a deadly game of cat and mouse in a secret war of espionage and intimidation. The nuclear balance between East and West was constantly shifting. This was a constant struggle to gain technological advantage, and the Soviets developed submarines that were ever more sophisticated - bigger, faster and more luxurious - than any developed by the West. For over 40 years the details of this tense stand-off have been a closely guarded secret. Now submariners from all three navies are able to talk more openly than ever before. They reveal how the underwater arms race took ballistic missiles beneath the Arctic ice, and they remember how it nearly ended in nuclear disaster at sea.