"What do they call you up there [in Philadelphia]?" -- "They call me Mr. Tibbs."
On a hot night in Sparta, Mississippi, Virgil Tibbs finds himself waiting for a train after a visit with his mother. All of a sudden, the town's police chief shows up and hauls him off to jail on suspicion of murder -- all because of the colour of his skin.
Based on the novel written by John Ball in 1965, In the Heat of the Night is a movie about a black detective from Philadelphia, played by Sidney Poitier, and a racist white police chief, played by Rod Steiger, who end up trying to solve a murder case together. Wealthy businessman Mr. Colbert from Chicago, has been in Sparta for the building of a new factory when he shows up dead. Chief Gillespie is in charge of the case. In his haste to pin the blame, he picks up Virgil Tibbs. Calling the black man "boy", he is shocked to hear that Virgil is a police detective back home. Gillespie asks: "What do they call you up there [in Philadelphia]?" And Virgil responds with: "They call me Mr. Tibbs."
Back at the police station, Chief Gillespie phones the Philadelphia police department to varify Tibbs' story. The Philadelphia chief suggests that Virgil stay in Sparta to help Gillespie solve the murder. Begrudgingly, Virgil agrees. It soon becomes apparent that Mr. Tibbs is more qualified to take the case than Mr. Gillespie, the latter wrongly accusing more than one suspect.
The first suspect is questioned by Virgil who realizes he can't be the murderer. The victim's widow overhears the questioning and realizes this detective from Philadelphia knows his stuff. She insists that he be named head of the investigation; otherwise, she will halt work on the new factory.
The next suspect is a wealthy plantation owner named Mr. Endicott. Resentful of Mr. Tibbs' questioning simply because of his race, he slaps the detective who in turn slaps him back. Endicott sends hooligans to attack Tibbs who is rescued by Gillespie.
Gillespie and Tibbs decide to accompany Sam Wood, the officer on duty the night of the murder, on his usual route. The police chief immediately suspects Wood of the murder when he changes his route. Also, Wood made a big deposit at the bank the day after the murder. On the route, the three men stop at a diner where the counterman refuses to serve Tibbs due to his colour. Back at the police station, Gillespie charges Wood with the murder. To further darken his reputation, local Mr. Purdy presses charges agains Wood for impregnating his 16 year old sister.
In the meantime, Mr. Tibbs clears Mr. Wood of the charges, explaining that he could not drive two cars, the police cruiser and the victim's vehicle, at the same time. Acting on a hunch, Mr. Tibbs visits a backroom abortionist and finds Mr. Purdy's sister there with her boyfriend -- the racist counterman at the diner. Mr. Tibbs has Mr. Purdy arrested, the latter confessing to the crime, saying he robbed Mr. Colbert (and accidentally killed him) to use the money for the abortion.
With the crime officially solved, the police chief drives the Philadelphia detective back to the train station. But this time, instead of calling him boy, he calls him Mister -- Mister Tibbs, that is.