In 1934, four brilliant and idealistic young men at Cambridge University are recruited to spy for Russia. Fuelled by youthful idealism, a passion for communism, a talent for lying and a hatred for Hitler and fascism, the four take huge personal risks to pass Britain's biggest secrets to Moscow. Across 20 years of spying and treachery, despite their personal journeys, the four are bound by their beliefs and secrets. They know more about each other than their wives or their lovers can ever know. And they are bound by the knowledge that there is one absolute centainty - they stand or fall together.