The 20th century's industrialization leaves the world overcrowded, polluted and stagnant. In 2022, with 40 million people in New York City alone, housing is dilapidated and overcrowded; homeless people fill the streets; the majority are unemployed, the few "lucky" people with jobs are only barely scraping by, and food and working technology is scarce. Most of the population survives on rations produced by the Soylent Corporation, whose newest product is Soylent Green, a green wafer advertised to contain "high-energy plankton" from the World Ocean, more nutritious and palatable than its predecessors "Red" and "Yellow", but in short supply.
New York City Police Department detective Frank Thorn lives with aged friend and "book" (a police analyst) Solomon "Sol" Roth. Roth remembers life before its current miserable state, and often waxes nostalgic. He is also well-educated and uses a small library of reference materials to assist Thorn. Investigating the murder of William R. Simonson, a wealthy businessman, Thorn questions Shirl, a concubine, and Tab Fielding, Simonson's bodyguard, who, when the murder took place, was escorting Shirl. Thorn searches Simonson's apartment for clues and enjoys Simonson's luxurious lifestyle, helping himself to Simonson's bourbon, fresh produce, and a steak Shirl purchased as a surprise for Simonson.
Thorn gives Roth the classified Soylent Oceanographic Survey Report, 2015 to 2019 he's found in Simonson's apartment. Roth's research reveals Simonson and the current governor of New York, Joseph Santini, were partners in a law firm, and that Simonson was also a member of the board of Soylent.
Thorn tells his lieutenant, Hatcher, that he suspects an assassination: nothing had been stolen from the apartment, its sophisticated alarm was not working for the first time in two years, and Simonson's bodyguard was absent. Thorn visits Fielding's apartment and interrogates Fielding's concubine, Martha, helping himself to a teaspoon of strawberry jam, later identified by Roth as too expensive a luxury for the concubine of a bodyguard to afford.
Shirl reveals that Simonson became troubled in the days before his death. Thorn questions a Catholic priest that Simonson visited; the priest, nearly catatonic from overwork, has trouble remembering Simonson and either cannot or will not describe the confession. Fielding later murders the priest.
Governor Santini closes the investigation, but Thorn ignores the directive and the Soylent Corporation dispatches Simonson's murderer to kill Thorn. He tracks Thorn to a ration distribution center where police officers are providing security. When the supply of Soylent Green is exhausted, the crowd riots. The assassin attempts to kill Thorn in the confusion, but is crushed by a "scoop" crowd-dispersion vehicle. Thorn threatens both Fielding and Martha to scare Fielding out of following him and returns to Shirl, telling her that all cities are like New York and the remaining pristine countryside is off-limits to all but the wealthy.
Roth takes Soylent's oceanographic reports to the Exchange, a group of researchers, who agree that the oceans no longer produce sufficient quantities of plankton from which Soylent Green is reputedly made, and infer that it is produced from human remains, the only supply of protein matching the known production rates. They also come to the conclusion that Simonson was murdered by Soylent out of fear that he might leak this news to the press, which would certainly cause a panic. Finally, they are determined to expose this once they have an eyewitness. Unable to live with this discovery, Roth seeks assisted suicide at a government clinic.
Thorn rushes to stop him, but arrives too late, and is mesmerized by the euthanasia process's visual and musical montage â a display of extinct forests, wild animals, rivers and ocean life. Before dying, Roth tells Thorn his discovery and begs him to expose the truth to the world through the researchers. Thorn stows himself aboard a garbage truck to the disposal center, where he sees human corpses converted into Soylent Green. Returning to make his report, he is hunted by Fielding and others to be turned into Soylent Green.
Thorn phones his precinct for backup but the precinct is engaged on a priority call. Thorn requests to be connected with Shirl, and to be "cut in" when the precinct is free. Thorn tells Shirl to stay with her apartment's new owner, and Shirl tells Thorn she wants to live with him, but the line is "cut in" and Thorn is connected to Hatcher. Thorn retreats into a cathedral filled with homeless people. He kills Fielding but is injured. When the police arrive, Thorn urges Hatcher to inform the Exchange and spread the word that "Soylent Green is people!"
Soylent Green was supposed to be made of what?
Plankton