In 2022, the population has grown to forty million people in New York City alone. Housing is dilapidated and overcrowded; homeless people fill the streets and line fire escapes and stairways. Food is scarce; most of the population survives on rations produced by the Soylent Corporation, whose newest product is Soylent Green, a small green wafer advertised to contain “high-energy plankton“. It is more nutritious and palatable than the other varieties, Red and Yellow, but is in short supply, which leads to food riots.
Protagonist Robert Thorn is a New York City Police Department detective living with his aged friend Solomon “Sol” Roth, a former scholar who searches the remnants of written records to help Thorn’s investigations. Thorn is assigned to investigate the murder of William R. Simonson, a director of the Soylent Corporation. Thorn visits Simonson’s apartment, where he helps himself to the latter’s food, liquor, bathroom, and books. He questions Shirl, an attractive concubine (referred to as “furniture”), and Simonson’s bodyguard, Tab Fielding, who claims to have been escorting Shirl shopping when the attack took place.
Returning to his apartment, Thorn gives Roth the book Soylent Oceanographic Survey Report, 2015 to 2019 taken from Simonson’s apartment. He visits the police station and tells his Lieutenant (Hatcher) that he suspects an assassination, because nothing was stolen from the apartment and the murder seemed professional, given that the apartment’s sophisticated alarm and monitoring electronics failed to detect the intruder and Fielding failed to prevent it. Continuing his investigation, Thorn visits Fielding’s apartment and questions Fielding’s mistress, Martha, again helping himself to items from their pantry, including some strawberry jam.
Thorn returns to his apartment and presents Roth with jam; Roth declares it too great a luxury for the concubine of a bodyguard. Thorn returns to question Shirl again; whereupon she tells him that Simonson became troubled in the days before his death. Thorn questions a priest whom Simonson had visited, but the addled priest at first fails to remember Simonson and is later unable to describe the latter’s confession. Fielding later murders the priest to silence him, suspecting the priest of revealing Simonson’s confession to Thorn.
When Thorn begins discovering why Simonson was murdered, New York State’s Governor Joseph Santini, once Simonson’s partner in a high-profile law firm, orders the investigation closed; but Thorn disobeys. The Soylent Corporation then dispatches Simonson’s murderer to kill Thorn. He tracks Thorn to a ration distribution effort where all of the police officers from the precinct are providing security. When the supply of Soylent Green runs out, the crowd riots and the assassin moves in for the kill during the confusion. Thorn sees him and escapes, and when the assassin gives chase he falls under a riot-control vehicle and is crushed.
Meanwhile, Roth has examined Soylent’s oceanographic reports and takes them to a like-minded group of researchers known as the “Exchange” to verify his conclusions. They agree; the oceans are barren, no longer producing the plankton from which Soylent Green is said to be made. In fact, it is made from human flesh. Unable to live with this discovery, Roth seeks assisted suicide at a government clinic in Madison Square Garden. When Thorn hears of this he rushes to stop him, but arrives in mid-procedure. Roth tells Thorn what he has discovered and begs him to follow his body to the processing center and report to the other scholars.
Thorn follows the trucks leaving the clinic and arrives at a heavily guarded facility where he sees human corpses being converted into Soylent Green. Returning to make his report, he is ambushed by Fielding with several compatriots. He phones his precinct for backup and then retreats into a cathedral filled with homeless people. He kills Fielding in the ensuing fight, but is seriously injured. When the police arrive, including Hatcher, Thorn urges Hatcher to spread the word that “Soylent Green is people!”