The English aristocrat's name is John Morgan and he is played by Richard Harris. He is captured while on a hunting trip and is taken to be a slave of the Sioux tribe. At first he is reluctant to adopt their way of life, but he is gradually initiated into their culture and becomes a respected member of the tribe.
He endures a painful initiation ritual, in which he is hung by hooks through his chest and must endure the pain without making a sound. This symbolic act earns him the respect of the tribe and he is given the name "Man Called Horse".
As he becomes more a part of the community, he discovers the Sioux's way of life and begins to understand the customs that differentiate the tribe from the white people who have encroached on their land. Morgan becomes increasingly committed to the Sioux culture, even marrying a Sioux woman and having a child with her.
As time goes on, Morgan becomes a leader in the tribe and starts to work to protect the Sioux from white settlers. He uses his knowledge of his own culture to help the Sioux plan attacks and defend their land.
The film, directed by Elliot Silverstein, was based on a short story by Dorothy M. Johnson and was well-received upon release in 1970. It spawned two sequels, "The Return of a Man Called Horse" and "Triumphs of a Man Called Horse".