The wicked go to hell

A cop receives a mission - to win the confidence of an enemy spy, currently in prison, and so to expose destroy his spy ring. What better way to allay his suspicions than for the policeman to enter the prison himself, posing as a criminal? So, Frank and Hal end up sharing a cell, but who is the spy and who is the cop? And who will win their claustrophobic game of cat and mouse?