In the year 2022, the population has grown to 40 million people in New York City alone. Most housing is dilapidated and overcrowded, and the homeless fill the streets and line the fire escapes, stairways of buildings, abandoned cars, subway platforms, etc. Unemployment is at around 50%. Summers are oppressively hot and humid with temperatures over 90F degrees during the day and night due to Earth's recent climate change resulting from the Greenhouse Effect. Food, as we know it in this present time, is a rare and expensive commodity. Most of the World's population survives on processed rations produced by the massive Soylent Corporation, including Soylent Red and Soylent Yellow, which are advertised as "high-energy vegetable concentrates". The newest product is Soylent Green: small green wafers which are advertised as being produced from "high-energy plankton". It is much more nutritious and palatable than the red and yellow varieties but, like most other foods, in short supply which often leads to weekly food riots.
Robert Thorn (Charlton Heston) is a New York City Police Department detective from the 14th Precinct who lives in a dilapidated, cramped one-room apartment with his aged friend and roommate, Solomon "Sol" Roth (Edward G. Robinson, in his last film). Roth is a former college professor whose job is to sort through the now-disordered remnants of written records and books to help Thorn's investigations. Roth and his like are known as "books." He tells Thorn about the time before the ecological disaster and population crisis of the early 2000s, when real food was plentiful, although Thorn is generally not interested in the stories, finding most of them too hard to believe.
Thorn is assigned to investigate the murder of William R. Simonson (Joseph Cotten), a 68-year-old wealthy lawyer living in a luxury high-rise apartment building called Chelsea Towers West. At the crime scene, Thorn first talks with the building superintendent Charles (Philip Stone) about discovering the body. Charles leads Thorn to the 22nd floor and to Simonson's apartment which is 22A. Thorn finds Simonson lying in a pool of blood after having been struck multiple times in the back of the head with a sharp object which is speculated to be a meat hook or an axe. Instead of looking for clues, the poorly paid detective helps himself to the wealthy man's food, liquor and books and even enjoys taking a shower (with real hot water and soap). He questions Shirl (Leigh Taylor-Young), an attractive 23-year-old "concubine" (euphemistically known as "furniture") who comes with the apartment, and Simonson's bodyguard, Tab Fielding (Chuck Connors), who claims that he was told to escort Shirl on a shopping trip when the attack took place. The sanitation crew led by Wagner arrives to take Simonson's dead body away, as Thorn leaves after collecting a statement from Fielding.
Returning to his apartment, Thorn gives Roth the Soylent Oceanographic Survey Report, 2015 to 2019, a two-volume work which he took from Simonson's apartment. Thorn returns to work at the 14th Precinct during that evening where they're hundreds of people lining up to collect death benefit money from recently diseased friends and family members. Thorn talks to his superior officer, Lieutenant Hatcher (Brock Peters), telling him that he suspects it may have been an assassination, since nothing was stolen from the apartment and the murder seemed professional. Thorn also finds it odd that the luxury apartment's sophisticated alarm and monitoring electronics, including the building's security cameras, happened to be inoperative on the night of the murder, and his bodyguard just happened to be out of the apartment at the time. Thorn confides in Lt. Hatcher that he suspects that Fielding might have had something to do with Simonson's murder.
The next day, Thorn stakes out Fielding's apartment building and sees him leave. Thorn lets himself into Fielding's apartment where he questions Fielding's live-in "furniture", Martha (Paula Kelly), about his work for Simonson. Thorn returns to his own apartment to eat an evening meal of the purloined food taken from Simonson's apartment that Roth prepares for them. Afterwards, Roth tells Thorn about the research he did on the murder victim and that Simonson was a member of the board of directors of the Soylent Corporation. When he presents Roth with a spoon of strawberry jam surreptitiously palmed from Fielding's apartment, Roth tastes it and declares that Fielding's "furniture" is eating some "$150-a-jar" strawberry jam, which is an out-of-place luxury for the mistress of a bodyguard.
Thorn returns to the Chelsea Towers West to question Shirl, who tells him that Simonson became deeply troubled in the days before his death, even taking her to church. Shirl is throwing a party in Simonson's vacant apartment for the other "furniture" girls of the building. Just then, Charles arrives and begins yelling and hitting the women for slacking off until Thorn arrives back in the living room to stop Charles from hurting the girls anymore. After sending Charles and the rest of the furniture girls away, Thorn returns to Shirl and they make love.
A little later that same evening, Thorn goes to a local Catholic Church (teeming with hundreds of homeless people) and attempts to question the priest (Lincoln Kirkpatrick) about Simonson's confession, but the priest is almost catatonic with exhaustion and has a hard time remembering Simonson, even though Simonson, as a rich man, would have stood out among the impoverished people who normally frequent the church. When the priest remembers Simonson, he tells Thorn the memory of what Simonson told him was haunting him, and is unable to describe what Simonson said to him because it was "too horrible". The next day, Fielding goes to the church and murders the priest to ensure he never talks again.
The next morning, as Thorn begins uncovering more evidence as to why Simonson was murdered, New York State's Governor, Joseph Santini (Whit Bissell), who was once Simonson's partner in a high-profile law firm and who is running for re-election (as shown in the campaign posters on such walls as that of Hatcher's office), instructs Hatcher to close the investigation. However, Thorn continues his investigation into the murder. When Governor Santini learns that Thorn refuses to close the case of the Simonson murder, he orders his chief of security, Donovan (Roy Jensen), and the head of security at Iannhop Corp. to have Thorn murdered.
When Thorn is on riot duty during the Tuesday distribution of Soylent Green rations, Simonson's murderer, Gilbert (Stephen Young), a local assassin-for-hire whom was the one that Donovan contracted to murder Simonson, attacks Thorn and fires several shots at him with a silenced pistol as a food riot begins. Thorn chases his attacker into the thick crowd as large dump trucks called "scoops" designed by the NYC Riot Police, arrive to pick up closely packed rioters and "scoop" them into the trucks. But before Thorn can capture Gilbert, who manages to wound Thorn by shooting him in his right calf, the assassin is crushed to death under the "scoop" of one of the riot control vehicles.
Thorn goes back to Fielding's apartment and beats him and his mistress Martha for the attempt on his life. Thorn then goes back to Shirl at Simonson's apartment where she removes the bullet and stitches his wound. Shirl tells Thorn that a new tenant will be arriving soon at the apartment and is hoping to move in, but she has romantic feelings for Thorn. Thorn tells Shirl that he will let her know if she wants to stay with him or the new tenant of Simonson's apartment. After resting for about an hour, Thorn leaves the apartment into the evening as sirens for the nightly curfew begin sounding. Sometime later a new potential tenant, a brash high-class businessman, arrives at the apartment and questions Shirl about herself and describes his daily details should he decide move in and want to keep her as "furniture".
Meanwhile, Roth examines Soylent's oceanographic reports at the Supreme Exchange, a library and gathering place for fellow "books." The "books" and Roth finally realize that the reports indicate a "horrible" truth which, despite reading it for themselves, they find nearly impossible to believe; Soylent Green isn't made from plankton, it's made from human bodies. Unable to live with what he has uncovered, Roth opts for assisted suicide at a government clinic (in the former Madison Square Garden, which had been converted to a clinic for mass euthanasia), a process referred to as "going home". As Roth is dying, he watches video clips of Earth long ago when animal (sheep, deer, and horses) and plant life was thriving and there was no pollution, while listening to light classical music.
Thorn returns to his apartment and finds a note from Roth that he is "going home". Thorn races to the clinic and forces the staff attendant (Dick Van Patten) to allow him to see and talk to Roth. During Roth's final moments, he tells Thorn the secret of Soylent Green, and begs him to follow his body to the processing center, and report back to the "Supreme Exchange".
Thorn sneaks into the basement of the assisted suicide facility, where he sees corpses being loaded onto waste disposal trucks. He secretly hitches a ride on one, which is driven to a heavily guarded waste disposal plant just outside the city. The black-clad sanitation drivers are escorted at gunpoint out of the truck where a white-clad plant worker takes over driving. The sanitation worker is escorted to another truck leaving the fortress-like facility to drive back to the city. Once inside the plant, Thorn sees how the corpses are processed into Soylent Green wafers. They are loaded onto a conveyer belt and driven through a large machine about several hundred yards long, and the Soylent Green wafers come out on the other end. But Thorn is spotted by two plant workers and is chased. Thorn kills one and subdues another before he escapes from the plant just when the alarm is sounded.
Thorn returns to the teeming city and heads for the Supreme Exchange to report what he found, but is ambushed by Fielding and three other gunmen (all of whom presumably work for the government and are trying to conceal the secret). Thorn manage to get to a police payphone and call Shirl to tell her that he loves her and to stay with the new tenant whom wants to rent Simonson's vacant apartment. Thorn also manages to place a second call to Lt. Hatcher at the 14th Precinct to tell him where he is and that he is being attacked, but then gets cut off when the assassins close in.
During a chase through the deserted streets and alleys (cleared of people because of the government-imposed dusk to dawn curfew), Thorn manages to kill all the gunmen chasing him except for Fielding who shoots Thorn in the back. He retreats into a cathedral filled with homeless people and Fielding follows him. After a desperate fight through throngs of sleeping homeless, Thorn kills Fielding by stabbing him with a rusted kitchen knife.
When police backup arrives at the church, the seriously wounded and nearly hysterical Thorn confides to Lt. Hatcher the horrible secret behind Soylent Green, while Hatcher looks on with disbelief. As Thorn is being carried away by the police on a stretcher to a local hospital, he urges to Hatcher to spread the word and shouts out to the spectators: "Soylent Green is PEOPLE!! We've got to stop them... SOMEHOW!!!"