Philby, Burgess and Maclean

In the late 1940s, British intelligence believes they have a Russian spy in their midst. They narrow the list to 5 or so individuals one of whom is Donald Maclean, who was assigned to the British Embassy in Washington and had full access to US atomic secrets. As they slowly build their case against Maclean, they assign Kim Philby to Washington as the senior liaison officer to the CIA. He is soon joined there by an old friend, Guy Burgess. When Philby gets wind that Maclean is about to be arrested, he dispatches Burgess to warn him and both defect to the Soviet Union. Philby is thoroughly investigated and while there is a general consensus that he too is a Russian spy, there is no concrete proof and, publicly at least, is cleared. Eight years later, in 1963, Philby also defects to the Soviet Union and it is revealed that he has been a KGB Colonel for over twenty years. Based on actual events.