The film begins with the murder of Phillip Colbert. Sam Wood, a local Sparta, Mississippi deputy, arrests Virgil Tibbs, a black man who is waiting for the 4:05am train to Memphis. Wood suspects that Virgil is the killer simply because he's black. But once Sheriff Gillespie questions Virgil he learns that he is a police detective from Philadelphia. Wood hadn't questioned Tibbs, he just arrested him.
Tibbs's Chief of police in Philadelphia confirms with Gillespie over the phone that he is in fact a police officer and a first rate homicide detective, and could be of use to the force in solving the murder of Colbert. Virgil objects but stays to helps Gillespie, as he tells his Chief that he doesn't have anything against the men in Sparta.
The story carries on to show the Sparta police force arresting innocent people in order to get a confession, but Virgil's high quality nature of detective work ensures that no innocent man will go to prison for this crime. The new detective pair of Tibbs and Gillespie go to search out a lead at the Endicott Cotton Co. The owner, Endicott grows orchids while black men and women pick cotton in his fields. Tibbs discovers a rare fertilizer that Endicott uses to grow his orchids which was near the corpse of Colbert. Endicott slaps Tibbs for implying he might have killed Colbert and Tibbs slaps him back before leaving the plantation.
Endicott threatens Virgil and sends men after him to hurt him. Gillespie gets Virgil out of the mix with the gang of white men and tells him he's got to leave town for his own safety. Virgil won't leave because he knows that he can solve this case. And after further detective work he begins to suspect that Wood, the police deputy is a primary suspect as he made a large deposit into his bank account the day after the murder. We learn that Delores a 16 year old has gotten pregnant by Wood and her brother, Purdy files charges against the deputy and Gillespie arrests him. Delores is then interrogated by Tibbs and Gillespie and this enrages Purdy as he cannot stand for a black man being present while his sister is questioned.
Purdy gets a group of white men to come and kill Virgil. All the while Virgil is able to clear Wood of the murder. Tibbs goes to an abortionist and discovers that someone paid for Delores to abort Wood's baby, and it's Ralph a young man from town. Virgil learns that Ralph killed Colbert when trying to rob him for the money to pay for the abortion. It wasn't Ralph's intent to kill the man, it happened by accident. Now Virgil is faced with Purdy's mob who've come to kill him but Ralph shoots Purdy dead. Ralph is then arrested and confesses to his crimes. With the murderer in custody, Virgil is finally able to board his train home to Philadelphia.
