A woman discovers the truth about her former lover from the diary that his first wife wrote to their son, Nicholas.

Detective Virgil Tibbs is caught up in the racial tension of the US South when he is arrested after the murder of a prominent businessman. Tibbs was simply waiting for his next train at the station in Sparta, Mississippi and the confusion is soon resolved but when local police chief Gillespie learns that Tibbs is the Philadelphia PD's number one homicide expert, he reluctantly asks for his assistance. The murdered man, Mr. Colbert, had come to Sparta from the North to build a new factory and his wife and business associates immediately point the finger at Endicott, the most powerful man in the county and the one who had the most to lose if a major new employer comes to the area. Tibbs' life is clearly in danger but he perseveres in a highly charged and racially explosive environment until the killer is found.