In the middle of the night in San Francisco, a group of six people wearing plastic masks pose as a maintenance crew in order to infiltrate the Century Furniture Factory. With silent efficiency, they kidnap factory manager John Bishop (Johnny Haymer), knock out guard George Morgan, and force Bishop to open the office vault, wherein lies their target: $4 million worth of heroin. Upon leaving, they blow up the building gate.Soon after, detective Lt. Virgil Tibbs (Sidney Poitier) and his partner, Lt. Jack Pecora (Gerald S. O'Loughlin), are called in to investigate and find Bishop dead at his desk. Tibbs begins by questioning Morgan, who was knocked out, about why the robbers would have blown up the gate when they could have made their escape without opening the gate. Assistant manager William Martin (Graham Jarvis) is also questioned, but has no idea why anyone would have killed Bishop, who has no obvious enemies.Upon returning to their office, the police learn that Bishop was shot with two guns, execution-style; that the robbers entered from outside a window five stories high; and that they had only four minutes after tripping the alarms to exit the building. Meanwhile, corporate executives Bob Alford (Fred Beir) and Zach Mills (John Lasell), the leaders of the drug syndicate that supplied Bishop with the heroin, speak to their colleagues in Istanbul and Paris. Using code, they inform them that the heroin has been stolen and Bishop killed.At home that night, Tibbs is bantering with his wife Valerie (Barbara McNair) and their children, Andy and Ginny, when he receives an anonymous phone call asking him to walk alone along a city street at a certain hour. Instructing Pecora to come after him in 30 minutes, Tibbs does as he is told and is soon ushered into an apartment where the robbers have gathered. Leader Juan Mendoza (Raul Julia) introduces Rev. Dave Thomas (Billy 'Green' Bush), Annie Sekido, Charlie Blossom, Stacy Baker and Joe Peralez, who admit they robbed the factory but insist that they did not murder Bishop. The vigilante group, each member of which has lost a loved one to drug addiction or overdose, wants to destroy "the organization." When they ask for Tibbs's help, pointing out that they deliberately blew up the gate in order to tip off the authorities, he refuses, stating that he cannot risk his career by banding with criminals without the knowledge of the police force. When he returns to Pecora, however, Tibbs says nothing about the group.That night, narcotics captain Grayson shoots himself, causing the men in the station to assume he was corrupt. After Bishop's wife lies to the police that she drove her husband to the factory the night of his murder, Tibbs realizes that she is being pressured not to admit that he was kidnapped. He arranges to meet Dave, Annie and Juan on a ferry, where he reluctantly acknowledges that they are not the killers and agrees to work with them, as long as they do not make a move without his approval.Tibbs re-questions Morgan, and when the guard refuses to admit that another car came in after the robbers broke in, Tibbs takes him in for further questioning. On way to the station, however, a car drives by and the passenger shoots Morgan, killing him before he can talk.Back at the station, Pecora wonders why Tibbs did not take him along to interrogate Morgan, but Tibbs deflects his questions and visits Morgan's widow Gloria to question her. She reveals that Morgan was rich, suggesting that Bishop was paying him off.Later, two toughs beat up Joe and offer him half a million dollars to meet them at Washington Square with information about where the heroin is hidden. Joe tells Juan, who urges him to meet the thugs in order to glean more information about them.Meanwhile, Pecora suggests that Tibbs call in the narcotics squad to help with the case, but Tibbs manages to hold him off. Joe goes to the designated meeting place and, while Juan and Annie watch nervously, a car arrives and takes him away. He shows the thugs a sample of the heroin and asks for $5 million, and in response, they threaten his life and order him to meet them that night. Annie, fearful for her friends, informs Tibbs, who instructs her to convince them that they must include him in their dealings, for their own safety. They arrange for Annie to call the station and anonymously tip off Pecora to the meeting point that night. As she hangs up the phone, however, the thugs follow her, and despite her efforts to elude them, they trap her in her car. Meanwhile, Jessop, the head of the narcotics division, assigns four men to help Tibbs and Pecora that night.The undercover police tail Joe, but when no one shows up at the meeting place, Tibbs is excoriated for wasting their time. Soon, Tibbs hears that Annie's dead body has been found, and meets the remaining five men to tell them that, because Annie was tortured to inform on them, they have been exposed and must flee. Tibbs and Pecora then question Martin again and, upon running through the complex alarm system with him, deduce that the murderers must have had an outside accomplice.Soon after, Tibbs learns from Dave that Joe's apartment is being watched by the thugs, and Tibbs rushes there, arriving just as the thugs run over Joe with their car. Although Tibbs arrests one of the men, the severely injured Joe blames the policeman for failing to protect him. When Jessop arrives at the hospital to question Joe, he learns of Tibbs's involvement, and soon Tibbs is called in to his superior, Capt. Stacy, and suspended for withholding information from the department. As Tibbs leaves, Jessop caustically informs him that Grayson killed himself not because he was corrupt but because he was dying of leukemia. Tibbs, unwilling to abandon the surviving vigilantes, asks his friend, the lawyer Sgt. Leo Chassman, for help in investigating Century. Chassman learns that the company has myriad "dummy" corporations as stockholders, and informs Tibbs about its complex financial workings.Meanwhile, Juan calls his friend Benjy, a small-time dealer, and offers to exchange the heroin for half a million dollars, on a crowded street. As Juan has hoped, Benjy arranges for the transfer with Alford and Mills, who arrive on the street to supervise the transaction. Just as Benjy and Juan swap bags, Charlie grabs the bag of heroin and runs off. While Tibbs's men, whom Juan has informed about the meeting, arrest Benjy, the organization's men chase Charlie into the unfinished subway system, followed by Tibbs. Together, Charlie and Tibbs shoot some of the thugs, and when the police arrive to finish the job, Tibbs runs off so they will not know he has been involved. He goes to Gloria's house and there casually reveals that he knows, based on the fact that Chassman's research has revealed that she is a major, wealthy stockholder in Century, who worked as a drug runner for the organization and facilitated Bishop's murder by tripping the alarms for the killers.After letting Pecora into the house, Tibbs advises Gloria to identify her bosses before they kill her, then uncovers a stash of heroin hidden inside a cut of beef in her refrigerator. Trapped, Gloria admits that Alford and Mills were behind the murder. The two men are arrested, but as they are being transported to testify, a sniper shoots them both before they can talk. Realizing that the organization will continue its dirty dealings, the reinstated Tibbs accepts a ride home from Pecora.