It's 1959 and Truman Capote (PHILIP SEYMOUR HOFFMAN) is a famous author and socialite who's at the top of his game. Having seen a brief story in the New York Times about a murder in America's heartland, Truman thinks it might make for an interesting article for his work in the New Yorker. Traveling to Kansas with his tenacious assistant and fellow writer Nelle Harper Lee (CATHERINE KEENER), Truman meets local sheriff Alvin Dewey (CHRIS COOPER) whose friend and family were murdered by two drifters.
As Truman and Nelle poke around for information, Dewey and his men eventually capture the two suspects, Perry Smith (CLIFTON COLLINS JR.) and Richard Hickock (MARK PELLEGRINO) who were mistakenly informed that a great deal of money was hidden in the house. Unable to find that, they murdered the family and then took off.
With the men now in custody and having bribed the local warden for access to them, Truman begins interviewing Perry, the more amiable of the two. Getting more information on the suspects and what occurred, Truman finds himself drawn to Perry, sensing something of a kindred spirit in him, albeit one whose life circumstances put him on a decidedly different course.
Accordingly, and after telling his editor that the story is too big for just a magazine article, Truman then sets out to write the world's first non-fiction novel about the murders and suspects. His time spent on the story and getting assistance for the suspects' defense, however, begins to put a bit of a strain on Truman's relationship with his gay lover/companion Jack Dunphy (BRUCE GREENWOOD).
With the years passing and the case drawing out even longer, Truman becomes obsessed with his work. Yet, he finds himself torn between his affection for Perry and needing to manipulate him for his novel that can't be finished until the case and the fate of the two killers is finally resolved.