Charles Julius Guiteau was an American man who assassinated James A. Garfield, the 20th president of the United States, in 1881. From Wikipedia
The series adapts Candice Millard’s Destiny of the Republic with Michael Shannon as Garfield, Matthew Macfadyen as Guiteau, Mike Makowsky as creator, Matt Ross directing, David Benioff, D. B. Weiss, Bernie Caulfield as executive producers.