Nat Geo's "Genius" ends with a bang as the world-renowned physicist Albert Einstein gets involved with the first atomic bombs and the FBI tries desperately to catch him committing communist acts of crime. 

The FBI obsessed over Einstein for the last two decades of the scientist's life, keeping a secret dossier with more than 1,400 pages of notes about his "radical" behavior until the day he died. 

Meanwhile, in the new episode, Einstein finally becomes a U.S. citizen and becomes involved with American politics during World War II, persuading President Franklin D. Roosevelt to pursue a nuclear program before Nazi Germany could build the first atomic bomb. 

Amidst his newfound political involvements, Einstein also continues his research at Princeton University in New Jersey, working to unify relativity, electromagnetism and quantum mechanics mathematically with what he called the "unified field theory." [The 'Genius' of Albert Einstein on Nat Geo Channel (Photos)]

Albert Einstein (Geoffrey Rush) smokes a cigar with President Franklin Delano Roosevelt (Mitchell Mullen) in National Geographic's "Genius."
Albert Einstein (Geoffrey Rush) smokes a cigar with President Franklin Delano Roosevelt (Mitchell Mullen) in National Geographic's "Genius."
Credit: Dusan Martincek/National Geographic

As the aging physicist begins to lose some of his mental acuity, he also copes with the loss of multiple loved ones. Even after becoming a grandparent, Einstein was never very involved with his children's lives. He finally reconciles with his estranged family before time runs out, but his work was left unfinished. 

Einstein kept trying to solve his elusive unified field theory through the very last hours of his life. At his bedside in the hospital was a notebook containing 12 pages of calculations — many of them crossed out or scribbled over — that Einstein hoped could bring the world one step closer to understanding the universe. 

Albert Einstein (Geoffrey Rush) takes a stroll through Princeton with Niels Bohr (David Dencik) in National Geographic's "Genius."
Albert Einstein (Geoffrey Rush) takes a stroll through Princeton with Niels Bohr (David Dencik) in National Geographic's "Genius."
Credit: Dusan Martincek/National Geographic

Unfortunately, Einstein wasn't able to unify the fundamental forces before his death on April 17, 1955. He was 76 years old. But researchers today are still working to come up with the same kind of unified theory Einstein had worked so tirelessly to conceptualize during the last three decades of his life. This has led modern physicists to pursue a concepts like the "theory of everything" and string theory

See how the last years of Einstein's life played out with the 2-hour season finale of "Genius" tonight (June 20) at 9 p.m. Eastern Time, 8 p.m. Central Time on the National Geographic Channel. 

Email Hanneke Weitering at hweitering@space.com or follow her @hannekescience. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.