The movie follows the careers of J. Edgar Hoover (Leonardo Dicaprio), Clyde Tolson (Armie Hammer) and Helen Glady (Noami Watts).
Hoover is a workaholic who, throughout the persuing decades, will stop at nothing to hunt down communinists. Also, he is blackmailing the Kennedys and Martin Luther King Jr. into bending to his demands.
Hoover and Tolson were possible lovers. Tolson at least more willing to accept his sexuality than Hoover.
When Glady is informed of Hoover's death, she immediately goes on destroying the secrets files Hoover collected for decades. There are very little records that still survive.
Thanks Michael R!