Season 21 aired between 26 Jan 2009 and 11 May 2009, 9 episodes.
Episodes
E01
The Trials of J. Robert Oppenheimer
26 Jan 2009
E02
The Polio Crusade
02 Feb 2009
E03
The Assassination of Abraham Lincoln
09 Feb 2009
E04
A Class Apart
23 Feb 2009
E05
We Shall Remain (1): After the Mayflower
13 Apr 2009
E06
We Shall Remain (2): Tecumseh's Vision
20 Apr 2009
E07
We Shall Remain (3): Trail of Tears
27 Apr 2009
E08
We Shall Remain (4): Geronimo
04 May 2009
E09
We Shall Remain (5): Wounded Knee
11 May 2009