Truman Capote (Philip Seymour Hoffman) is riding high from the success of his novel Breakfast at Tiffany's. He is known as a deliciously entertaining raconteur amongst the Jet set. One day in New York City, he reads about a brutal crime in Holcomb, Kansas where four members of a well-to-do family were murdered. Capote is intrigued with the possible impact of this tragedy on the community, and asks William Shawn (Bob Balaban), the editor of the New Yorker, to begin work on a magazine article. On the train with Nelle Harper Lee (Catherine Keener), his research assistant, he pulls a stunt which offers a telling insight into his personality. A porter opens the door to their compartment and praises Capote for his last work, saying it was even better than his first. But Nelle realizes that her friend paid the man for the compliment. Another sign of his need to be seen by all as special is Capote's flaunting of his fashionable clothing once he arrives in Kansas, where to the ordinary Midwestern folk he seems like a visitor from another planet.

The first clue to Capote's realization that he is onto a bigger story than he thought is when he goes to the funeral home where the four closed caskets are housed. He opens them up and peers inside. When Nelle gets an interview with Laura Kinney (Allie Mickelson), the seventeen-year old who found the Clutter family bodies, Capote wins her cooperation by sharing with her some intimate details of his lifelong struggle as an outsider who was constantly misunderstood by others. The young girl identifies with him and hands over Nancy Clutter's diary. At the invitation of Marie Dewey (Amy Ryan), the wife of Alvin Dewey (Chris Cooper), the lead agent for the Kansas Bureau of Investigation, they spend an evening together. Capote looks at the photos from the murder scene and is shocked and mystified. Some time later, the two accused killers are captured: Perry Smith (Clifton Collins, Jr) and Richard Hickock (Mark Pellegrino). After briefly chatting with Smith, Capote calls Shawn and tells him that he is going to do a book. Richard Avedon (Adam Kimmel) arrives to photograph the two men. In 1960, Smith and Hickock are found guilty and sentenced to death.

This mesmerizing film is directed by Bennett Miller (The Cruise) and written by Dan Futterman based on the book by Gerald Clark. Philip Seymour Hoffman gives one of the most stunning performances of the year as the egocentric, entertaining, and manipulative writer whose need to be seen as special makes him into a devious and disturbing man. The last half of the film focuses on the macabre relationship that develops between Capote and Smith that involves many visits to him on Death Row at Kansas State Penitentiary in Lansing. He gets them a new lawyer to appeal their case. The writer rescues Smith from a hunger strike and convinces him to hand over his notebooks. Meanwhile, Capote's lover Jack (Bruce Greenwood) is jealous of all the attention lavished on Smith. Capote says, "It’s as if Perry and I started life in the same house. One day he stood up and walked out the back door while I walked out the front."

Shawn is excited when he reads the first drafts of In Cold Blood and sets up a public reading of this breakthrough in what Capote calls "the non-fiction novel." Every time Smith asks about the book or its title, his friend lies to him. After the murderers are given several stays of execution, Capote convinces Smith to reveal what really happened the night of the murders. The closing episodes of the film are deeply moving and disturbing at the same time. They reveal the dark side of Capote and of Smith, the two outsiders whose lives are so different and yet so similar in some very strange ways.